Resumen de Conmutacion y Ruteo i

© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public 1

CONMUTACION AND ROUTING I.MSc. MARCELO DONOSO V.


Introduction to Routing and Packet Forwarding

Concepts – Chapter 1

3© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Router as a Computer§ Describe the basic purpose of a router

-Computers that specialize in sending packets over the data network. They are responsible for interconnecting networks by selecting the best path for a packet to travel and forwarding packets to their destination

§ Routers are the network center-Routers generally have 2 connections:

-WAN connection (Connection to ISP)-LAN connection


Router as a Computer§ Data is sent in form of packets between 2 end devices

§ Routers are used to direct packet to its destination


Router as a Computer§ Routers examine a packet’s destination IP address and

determine the best path by enlisting the aid of a routing table


Router as a Computer§ Router components and their functions”

§CPU - Executes operating system instructions§Random access memory (RAM) - Contains the running copy of configuration file. Stores routing table. RAM contents lost when power is off§Read-only memory (ROM) - Holds diagnostic software used when router is powered up. Stores the router’s bootstrap program.§Non-volatile RAM (NVRAM) - Stores startup configuration. This may include IP addresses (Routing protocol, Hostname of router)§Flash memory - Contains the operating system (Cisco IOS)§Interfaces - There exist multiple physical interfaces that are used to connect network. Examples of interface types:

-Ethernet / fast Ethernet interfaces-Serial interfaces-Management interfaces


Router as a Computer§ Router components


Router as a Computer§ Major phases to the

router boot-up process§Test router hardware

Power-On Self Test (POST)Execute bootstrap loader

§Locate & load Cisco IOS software

-Locate IOS-Load IOS

§Locate & load startup configuration file or enter setup mode

-Bootstrap program looks for configuration file


Router as a Computer§ Verify the router boot-up process:

-The show version command is used to view information about the router during the bootup process. Information includes:§Platform model number§Image name & IOS version§Bootstrap version stored in ROM§Image file name & where it was loaded from§Number & type of interfaces§Amount of NVRAM§Amount of flash§Configuration register


Router as a Computer


Router as a Computer§ Router Interface is a physical connector that enables a

router to send or receive packets

§ Each interface connects to a separate network

§ Consist of socket or jack found on the outside of a router

§ Types of router interfaces:-Ethernet-Fastethernet-Serial-DSL-ISDN-Cable


Router as a Computer§ Two major groups of Router Interfaces

LAN Interfaces:

§Are used to connect router to LAN network§Has a layer 2 MAC address §Can be assigned a Layer 3 IP address§Usually consist of an RJ-45 jack

§ WAN Interfaces§Are used to connect routers to external networks that interconnect LANs.§Depending on the WAN technology, a layer 2 address may be used.§Uses a layer 3 IP address


Router as a Computer§ Routers and the Network Layer

Routers use destination IP address to forward packets§The path a packet takes is determined after a router consults information in the routing table.§After router determines the best path§Packet is encapsulated into a frame§Frame is then placed on network medium in form of Bits


Router as a Computer§ Routers Operate at Layers 1, 2 & 3

§Router receives a stream of encoded bits§Bits are decoded and passed to layer 2§Router de-encapsulates the frame§Remaining packet passed up to layer 3

-Routing decision made at this layer by examining destination IP address

§Packet is then re-encapsulated & sent out outbound interface


Configure Devices and Apply Addresses§ Implementing Basic Addressing Schemes

§ When designing a new network or mapping an existing network you must provide the following information in the form of a document:

-Topology drawing that Illustrates physical connectivity–Address table that provides the following information:§Device name§Interfaces used§IP addresses§Default gateway


Configure Devices and Apply Addresses§ Basic Router Configuration§ A basic router configuration should contain the following:

-Router name - Host name should be unique-Banner - At a minimum, banner should warn against unauthorized use-Passwords - Use strong passwords-Interface configurations - Specify interface type, IP address and subnet mask. Describe purpose of interface. Issue no shutdown command. If DCE serial interface issue clock rate command.

§ After entering in the basic configuration the following tasks should be completed

-Verify basic configuration and router operations. -Save the changes on a router


Configure Devices and Apply Addresses


Configure Devices and Apply Addresses§ Verify Basic Router Configuration

-Issue the show running-config command-Save the basic router configuration by Issuing the copy running-config startup-config command-Additional commands that will enable you to further verify router configuration are:§Show running-config - Displays configuration currently in RAM§Show startup-config - Displays configuration file NVRAM§Show IP route - Displays routing table§Show interfaces - Displays all interface configurations§Show IP int brief - Displays abbreviated interface configuration information


Routing Table Structure§ Routing Table is stored in ram and contains information

about:§Directly connected networks - this occurs when a device is connected to another router interface§Remotely connected networks - this is a network that is not directly connected to a particular router§Detailed information about the networks include source of information, network address & subnet mask, and Ip address of next-hop router

§ Show ip route command is used to view a routing table


Routing Table Structure§ Adding a connected network to the routing table

-Router interfaces§Each router interface is a member of a different network§Activated using the no shutdown command§In order for static and dynamic routes to exist in routing table you must have directly connected networks


Routing Table Structure§ Static routes in the routing table

-Includes: network address and subnet mask and IP address of next hop router or exit interface-Denoted with the code S in the routing table-Routing tables must contain directly connected networks used to connect remote networks before static or dynamic routing can be used

§ When to use static routes-When network only consists of a few routers-Network is connected to internet only through one ISP-Hub & spoke topology is used on a large network


Routing Table Structure§ Connected and Static routes


Routing Table Structure§ Dynamic routing protocols

-Used to add remote networks to a routing table-Are used to discover networks-Are used to update and maintain routing tables

§ Automatic network discovery-Routers are able discover new networks by sharing routing table information


Routing Table Structure§ Maintaining routing tables

-Dynamic routing protocols are used to share routing information with other router & to maintain and up date their own routing table.

§ IP routing protocols. Example of routing protocols include:-RIP-IGRP-EIGRP-OSPF


Routing Table Structure§ Routing Table Principles

-3 principles regarding routing tables: §Every router makes its decisions alone, based on the information it has in its routing table.§Different routing table may contain different information§ A routing table can tell how to get to a destination but not how to get back


Routing Table Structure§ Effects of the 3 Routing Table Principles

-Packets are forwarded through the network from one router to another, on a hop by hop basis.-Packets can take path “X” to a destination but return via path “Y” (Asymmetric routing).


Router Paths and Packet Switching§ Internet Protocol (IP) packet format contains fields that

provide information about the packet and the sending and receiving hosts

§ Fields that are importance for students:-Destination IP address-Source IP address-Version & TTL-IP header length-Precedence & type of service-Packet length


Router Paths and Packet Switching§ MAC Layer Frame Format

§ MAC Frames are also divided into fields. They include:-Preamble-Start of frame delimiter-Destination MAC address-Source MAC address-Type/length-Data and pad-Frame check sequence


Router Paths and Packet Switching§ A Metric is a numerical value used by routing protocols help

determine the best path to a destination–The smaller the metric value the better the path

§ 2 types of metrics used by routing protocols are:-Hop count - this is the number of routers a packet must travel through to get to its destination-Bandwidth - this is the “speed” of a link also known as the data capacity of a link


Router Paths and Packet Switching§ Equal cost metric is a condition where a router has multiple paths

to the same destination that all have the same metric

§ To solve this dilemma, a router will use Equal Cost Load Balancing. This means the router sends packets over the multiple exit interfaces listed in the routing table.


Router Paths and Packet Switching§ Path determination is a process used by a router to pick the best

path to a destination§ One of 3 path determinations results from searching for the best

pathDirectly connected networkRemote networkNo route determined


Router Paths and Packet Switching§ Switching Function of Router is the process used by a

router to switch a packet from an incoming interface to an outgoing interface on the same router.

-A packet received by a router will do the following:§Strips off layer 2 headers. §Examines destination IP address located in Layer 3 header to find best route to destination.§Re-encapsulates layer 3 packet into layer 2 frame. §Forwards frame out exit interface.


Router Paths and Packet Switching§ As a packet travels from one networking device to another

-The Source and Destination IP addresses NEVER change-The Source & Destination MAC addresses CHANGE as packet is forwarded from one router to the next.-TTL field decrement by one until a value of zero is reached at which point router discards packet (prevents packets from endlessly traversing the network)


Router Paths and Packet Switching§ Path determination and switching function details. PC1

Wants to send something to PC 2 here is part of what happens

Step 1 - PC1 encapsulates packet into a frame. Frame contains R1’s destination MAC address


Router Paths and Packet SwitchingStep 2 - R1 receives Ethernet frame. §R1 sees that destination MAC address matches its own MAC. §R1 then strips off Ethernet frame. §R1 Examines destination IP. §R1 consults routing table looking for destination IP. §After finding destination IP in routing table, R1 now looks up next hop IP address.§R1 re-encapsulates IP packet with a new Ethernet frame.§R1 forwards Ethernet packet out Fa0/1 interface.


Router Paths and Packet Switching


Router Paths and Packet Switching§ Path determination and switching function details. PC1 Wants to

send something to PC 2 here is part of what happensStep 3 - Packet arrives at R2§R2 receives Ethernet frame§R2 sees that destination MAC address matches its own MAC§R2 then strips off Ethernet frame§R2 Examines destination IP§R2 consults routing table looking for destination IP§After finding destination IP in routing table, R2 now looks up next hop IP address§R2 re-encapsulates IP packet with a new data link frame§R2 forwards Ethernet packet out S0/0 interface


Router Paths and Packet Switching§ Path determination and switching function details. PC1 Wants to

send something to PC 2 here is part of what happensStep 4 - Packet arrives at R3 §R3 receives PPP frame§R3 then strips off PPP frame§R3 Examines destination IP §R3 consults routing table looking for destination IP§After finding destination IP in routing table, R3 is directly connected to destination via its fast Ethernet interface§R3 re-encapsulates IP packet with a new Ethernet frame§R3 forwards Ethernet packet out Fa0/0 interface

Step 5 - IP packet arrives at PC2. Frame is decapsulated & processed by upper layer protocols.


Summary§ Routers are computers that specialize in sending data over a network.§ Routers are composed of:

-Hardware i.e. CPU, Memory, System bus, Interfaces-Software used to direct the routing process§IOS§Configuration file

§ Routers need to be configured. Basic configuration consists of:-Router name-Router banner-Password(s)-Interface configurations i.e. IP address and subnet mask

§ Routing tables contain the following information-Directly connected networks-Remotely connected networks-Network addresses and subnet masks-IP address of next hop address


Summary§ Routers determine a packets path to its destination by

doing the following§Receiving an encapsulated frame & examining destination MAC address.§If the MAC address matches then Frame is de-encapsulated so that router can examine the destination IP address.§If destination IP address is in routing table or there is a static route then Router determines next hop IP address. Router will re-encapsulate packet with appropriate layer 2 frame and send it out to next destination.§Process continues until packet reaches destination.§Note - only the MAC addresses will change the source and destination IP addresses do not change.


Static Routing



Objectives§ Define the general role a router plays in networks.

§ Describe the directly connected networks, different router interfaces

§ Examine directly connected networks in the routing table and use the CDP protocol

§ Describe static routes with exit interfaces

§ Describe summary and default route

§ Examine how packets get forwarded when using static routes

§ Identify how to manage and troubleshoot static routes


General Role of the Router§ Functions of a Router

Best Path SelectionsForwarding packets to destination

§ Introducing the Topology3 1800 series routers connected via WAN linksEach router connected to a LAN represented by a switch and a PC


General Role of the Router§ Connections of a Router for WAN

-A router has a DB-60 port that can support 5 different cabling standards

§ Connections of a Router for Ethernet-2 types of connectors can be used: Straight through and Cross-over §Straight through used to connect:

-Switch-to-Router, Switch-to-PC, Router-to-Server, Hub-to-PC, Hub-to-Server

§Cross-over used to connect: -Switch-to-Switch, PC-to-PC, Switch-to-Hub, Hub-to-Hub, Router-to-Router


Interfaces§ Examining Router Interfaces

-Show IP router command – used to view routing table-Show Interfaces command – used to show status of an

interface-Show IP Interface brief command – used to show a portion of

the interface information-Show running-config command – used to show configuration

file in RAM


Interfaces§ Configuring an Ethernet interface

-By default all serial and Ethernet interfaces are down-To enable an interface use the No Shutdown command


Interfaces§ Verifying Ethernet interface

-Show interfaces for fastEthernet 0/0 – command used to show status of fast Ethernet port

-Show ip interface brief-Show running-config

§ Ethernet interfaces participate in ARP


Interfaces§ Configuring a Serial interface

-Enter interface configuration mode-Enter in the ip address and subnet mask-Enter in the no shutdown command

§ Example:-R1(config)#interface serial 0/0-R1(config-if)#ip address 172.16.2.1 255.255.255.0-R1(config-if)#no shutdown


Interfaces§ Examining Router Interfaces

-Physically connecting a WAN Interface. -A WAN Physical Layer connection has sides:§Data Circuit-terminating Equipment (DCE) – This is the service provider. CSU/DSU is a DCE device. §Data Terminal Equipment (DTE) – Typically the router is the DTE device.


Interfaces§ Configuring serial links in a lab environment

§One side of a serial connection must be considered a DCE§This requires placing a clocking signal – use the clock rate command.§Example:

-R1(config)#interface serial 0/0-R1(config-if)#clockrate 64000

§Serial Interfaces require a clock signal to control the timing of the communcations.


Routing Table and CDP Protocol§ Purpose of the debug ip routing command

§Allows you to view changes that the router performs when adding or removing routes§Example:

-R2#debug ip routing-IP routing debugging is on


Routing Table and CDP Protocol§ To configure an Ethernet interface

§Example:-R2(config)#interface fastethernet 0/0-R2(config-if)#ip address 172.16.1.1 255.255.255.0-R2(config-if)#no shutdown


Routing Table and CDP Protocol§ When a router only has its interfaces configured & no

other routing protocols are configured then:-The routing table contains only the directly connected networks-Only devices on the directly connected networks are reachable


Routing Table and CDP Protocol


Routing Table and CDP Protocol§ Checking each

route in turnThe pingcommand is used to check end to end connectivity


Routing Table and CDP Protocol§ Purpose of CDP

A layer 2 cisco proprietary tool used to gather information about other directly connected Cisco devices.

§ Concept of neighbors-2 types of neighbors§Layer 3 neighbors§Layer 2 neighbors


Routing Table and CDP Protocol§ CDP show commands

§Show cdp neighbors command-Displays the following information:§Neighbor device ID§Local interface§Holdtime value, in seconds§Neighbor device capability code§Neighbor hardware platform§Neighbor remote port ID

§Show cdp neighbors detail command -Useful in determining if an IP address configuration error


Routing Table and CDP Protocol§ Disabling CDP

To disable CDP globally use the following commandRouter(config)#no cdp run


Static Routes with Exit Interfaces§ Purpose of a static route

A manually configured route used when routing from a network to a stub network


Static Routes with Exit Interfaces§ IP route command

§To configure a static route use the following command: ip route§Example:

-Router(config)# ip route network-address subnet-mask {ip-address | exit-interface }


Static Routes with Exit Interfaces§ Dissecting static route syntax

§ip route - Static route command§172.16.1.0 – Destination network address §255.255.255.0 - Subnet mask of destination network§172.16.2.2 - Serial 0/0/0 interface IP address on R2, which is the "next-hop" to this network


Static Routes with Exit Interfaces§ Configuring routes to 2 or more remote networks

Use the following commands for R1-R1(config)#ip route 192.168.1.0 255.255.255.0 172.16.2.2-R1(config)#ip route 192.168.2.0 255.255.255.0 172.16.2.2


Static Routes with Exit Interfaces§ Zinin’s 3 routing principles

§Principle 1: "Every router makes its decision alone, based on the information it has in its own routing table.“§Principle 2: "The fact that one router has certain information in its routing table does not mean that other routers have the same information.“§Principle 3: "Routing information about a path from one network to another does not provide routing information about the reverse, or return path."


Static Routes with Exit Interfaces§ Using Zinin’s 3 routing principles, how would you

answer the following?-Would packets from PC1 reach their destination?

Yes, packets destined for 172.16.1.0/24 and 192.168.1.0/24 networks would reach their destination.

-Does this mean that any packets from these networks destined for 172.16.3.0/24 network will reach their destination?No, because neither R2 nor R3 router has a route to the

172.16.3.0/24 network.


Static Routes with Exit Interfaces§ Resolving to an Exit Interface

-Recursive route lookup - Occurs when the router has to perform multiple lookups in the routing table before forwarding a packet. A static route that forwards all packets to the next-hop IP address goes through the following process (reclusive route lookup)§The router first must match static route’s destination IP address with the Next hop address§The next hop address is then matched to an exit interface


Static Routes with Exit Interfaces§ Configuring a Static route with an Exit Interface

-Static routes configured with an exit interface are more efficientbecause the routing–The routing table can resolve the exit interface in a single search instead of 2 searches-Example of syntax require to configure a static route with an exit interface


Static Routes with Exit Interfaces§ Modifying Static routes

§Existing static routes cannot be modified. The old static route must be deleted by placing no in front of the ip route§Example:

-no ip route 192.168.2.0 255.255.255.0 172.16.2.2§A new static route must be rewritten in the configuration


Static Routes with Exit Interfaces§ Verifying the Static Route Configuration

-Use the following commands§Step 1 show running-config§Step 2 verify static route has been entered correctly§Step 3 show ip route§Step 4 verify route was configured in routing table§Step 5 issue ping command to verify packets can reach destination and that Return path is working


Static Routes with Exit Interfaces§ Ethernet interfaces and ARP.

– If a static route is configured on an Ethernet link-If the packet is sent to the next-hop router then…

the destination MAC address will be the address of the next hop’s Ethernet interfaceThis is found by the router consulting the ARP table.

If an entry isn’t found then an ARP request will be sent out


Summary and Default Route§ Summarizing routes reduces the size of the routing

table.

§ Route summarization is the process of combining a number of static routes into a single static route.


Summary and Default Route§ Configuring a summary route

Step 1: Delete the current static routeStep 2: Configure the summary static routeStep 3: Verify the new static route


Summary and Default Route§ Default Static Route

§This is a route that will match all packets. Stub routers that have a number of static routes all exiting the same interface are good candidates for a default route.

-Like route summarization this will help reduce the size of the routing table

§ Configuring a default static route§Similar to configuring a static route. Except that destination IP address and subnet mask are all zeros§Example:

-Router(config)#ip route 0.0.0.0 0.0.0.0 [exit-interface | ip-address ]


Summary and Default Route§ Static routes and subnet masks

The routing table lookup process will use the most specific match when comparing destination IP address and subnet mask

§ Default static routes and subnet masksSince the subnet mask used on a default static route is 0.0.0.0 all packets will match.


Static Routes and Packet Forwarding§ Packet forwarding

with static routes. (recall Zinin’s 3 routing principles)§ Router 1

Packet arrives on R1’s Fastethernet 0/0 interface

R1 does not have a route to the

destination network, 192.168.2.0/24R1 uses the default static route.


Static Routes and Packet Forwarding§ Packet forwarding with static routes. (recall Zinin’s 3

routing principles)

§ Router 2The packet arrives on the Serial 0/0/0 interface on R2.R2 has a static route to 192.168.2.0/24 out Serial0/0/1.


Static Routes and Packet Forwarding§ Packet forwarding with static routes. (recall Zinin’s 3

routing principles)

§ Router 3The packet arrives on the Serial0/0/1 interface on R3.R3 has a connected route to 192.168.2.0/24 out Fastethernet 0/1.


Static Routes and Packet Forwarding§ Troubleshooting a Missing Route

§ Tools that can be used to isolate routing problems include:

-Ping– tests end to end connectivity-Traceroute– used to discover all of the hops (routers) along the path between 2 points-Show IP route– used to display routing table & ascertain forwarding process-Show ip interface brief- used to show status of router interfaces-Show cdp neighbors detail– used to gather configuration information about directly connected neighbors


Static Routes and Packet Forwarding§ Solving a Missing Route

§ Finding a missing or mis-configured route requires methodically using the correct tools

-Start with PING. If ping fails then use traceroute to determine where packets are failing to arrive

§ Issue: show ip route to examine routing table.-If there is a problem with a mis-configured static route remove the static route then reconfigure the new static route


Summary§ Routers

-Operate at layer 3-Functions include best path selection & forwarding packets

§ Connecting NetworksWANs

Serial cables are connected to router serial ports. In the lab environment clock rates must be configured for DCE

LANsStraight through cables or cross over cables are used to connect to fastethernet port. (The type of cable used depends on what devices are being connected)

§ Cisco Discovery ProtocolA layer 2 proprietary protocolUsed to discover information about directly connected Cisco devices


Summary§ Static Routes

-This is a manually configured path that specifies how the router will get to a certain point using a certain path.

§ Summary static routes-This is several static routes that have been condensed into a single static route.

§ Default route-It is the route packets use if there is no other possible match for their destination in the routing table.

§ Forwarding of packets when static route is used-Zinin’s 3 routing principles describe how packets are forwarded

§ Troubleshooting static routes may require some of the following commands:

-Ping -Traceroute-Show IP route-Show ip interface brief -Show cdp neighbors detail


Introduction to Dynamic Routing Protocol



Objectives§ Describe the role of dynamic routing protocols and

place these protocols in the context of modern network design.

§ Identify several ways to classify routing protocols.

§ Describe how metrics are used by routing protocols and identify the metric types used by dynamic routing protocols.

§ Determine the administrative distance of a route and describe its importance in the routing process.

§ Identify the different elements of the routing table.


Dynamic Routing Protocols§ Function(s) of Dynamic Routing Protocols:

-Dynamically share information between routers.-Automatically update routing table when topology changes.-Determine best path to a destination.


Dynamic Routing Protocols§ The purpose of a dynamic routing protocol is to:

-Discover remote networks-Maintaining up-to-date routing information-Choosing the best path to destination networks-Ability to find a new best path if the current path is no longer available


Dynamic Routing Protocols§ Components of a routing protocol

AlgorithmIn the case of a routing protocol algorithms are used for facilitating routing information and best path determination

Routing protocol messagesThese are messages for discovering neighbors and exchange of routing information


Dynamic Routing Protocols§ Advantages of static routing

-It can backup multiple interfaces/networks on a router-Easy to configure-No extra resources are needed-More secure

§ Disadvantages of static routing-Network changes require manual reconfiguration -Does not scale well in large topologies


Classifying Routing Protocols§ Dynamic routing protocols are grouped according to

characteristics. Examples include:-RIP-IGRP-EIGRP-OSPF-IS-IS-BGP

§ Autonomous System is a group of routers under the control of a single authority.


Classifying Routing Protocols§ Types of routing protocols:

-Interior Gateway Protocols (IGP)-Exterior Gateway Protocols (EGP)


Classifying Routing Protocols§ Interior Gateway Routing Protocols (IGP)

-Used for routing inside an autonomous system & used to route within the individual networks themselves.-Examples: RIP, EIGRP, OSPF

§ Exterior Routing Protocols (EGP)-Used for routing between autonomous systems-Example: BGPv4


Classifying Routing Protocols§ IGP: Comparison of Distance Vector & Link State

Routing ProtocolsDistance vector

– routes are advertised as vectorsof distance & direction.

– incomplete view of networktopology.

–Generally, periodicupdates.

Link state– complete view of network

topology is created.– updates are not

periodic.


Classifying Routing Protocols


Classifying Routing Protocols

§ Classful routing protocols

Do NOT send subnet mask in routing updates

§ Classless routing protocols

Do send subnet mask in routing updates.


Classifying Routing Protocols§ Convergence is defined as when all routers’ routing

tables are at a state of consistency


Routing Protocols Metrics§ Metric

A value used by a routing protocol to determine which routes are better than others.


Routing Protocols Metrics§ Metrics used in IP routing protocols

-Bandwidth-Cost-Delay-Hop count-Load-Reliability


Routing Protocols Metrics§ The Metric Field in the

Routing Table

§ Metric used for each routing protocol

-RIP - hop count-IGRP & EIGRP -Bandwidth (used by default), Delay (used by default), Load, Reliability-IS-IS & OSPF – Cost, Bandwidth (Cisco’s implementation)


Routing Protocols Metrics§ Load balancing

This is the ability of a router to distribute packets among multiple same cost paths


Administrative Distance of a Route§ Purpose of a metric

It’s a calculated value used to determine the best path to a destination

§ Purpose of Administrative DistanceIt’s a numeric value that specifies the preference of a particular route


Administrative Distance of a Route§ Identifying the Administrative Distance (AD) in a

routing tableIt is the first number in the brackets in the routing table


Administrative Distance of a Route§ Dynamic Routing Protocols


Administrative Distance of a Route§ Directly connected routes

Have a default AD of 0

§ Static RoutesAdministrative distance of a static route has a default value of 1


Administrative Distance of a Route§ Directly connected routes

-Immediately appear in the routing table as soon as the interface is configured


Summary§ Dynamic routing protocols fulfill the following functions

-Dynamically share information between routers-Automatically update routing table when topology changes-Determine best path to a destination

§ Routing protocols are grouped as either-Interior gateway protocols (IGP)Or-Exterior gateway protocols(EGP)

§ Types of IGPs include-Classless routing protocols - these protocols include subnet mask in routing updates-Classful routing protocols - these protocols do not include subnet mask in routing update


Summary§ Metrics are used by dynamic routing protocols to calculate the

best path to a destination.§ Administrative distance is an integer value that is used to

indicate a router’s “trustworthiness”§ Components of a routing table include:

-Route source-Administrative distance-Metric


Distance Vector Routing Protocols

Chapter 4


Objectives§ Identify the characteristics of distance vector routing

protocols.

§ Describe the network discovery process of distance vector routing protocols using Routing Information Protocol (RIP).

§ Describe the processes to maintain accurate routing tables used by distance vector routing protocols.

§ Identify the conditions leading to a routing loop and explain the implications for router performance.

§ Recognize that distance vector routing protocols are in use today


Distance Vector Routing Protocols§ Examples of Distance Vector routing protocols:

§Routing Information Protocol (RIP)

§Interior Gateway Routing Protocol (IGRP)

§Enhanced Interior Gateway Routing Protocol(EIGRP)


Distance Vector Routing Protocols§ Distance Vector Technology

–The Meaning of Distance Vector:

•A router using distance vector routing protocols knows 2 things:

§Distance to final destination

§Vector, or direction, traffic should be directed


Distance Vector Routing ProtocolsCharacteristics of Distance Vector routing protocols:

§ Periodic updates§ Neighbors § Broadcast updates§ Entire routing table is included with routing update


Distance Vector Routing Protocols§ Routing Protocol Algorithm:

-Defined as a procedure for accomplishing a certain task


Distance Vector Routing ProtocolsRouting Protocol Characteristics

–Criteria used to compare routing protocols includes

§-Time to convergence §-Scalability §-Resource usage §-Implementation & maintenance


Distance Vector Routing Protocols


Network Discovery§ Router initial start up (Cold Starts)

-Initial network discovery§Directly connected networks are initially placed in

routing table


Network Discovery§ Initial Exchange of Routing Information

–If a routing protocol is configured then

-Routers will exchange routing information

§ Routing updates received from other routers

-Router checks update for new information§If there is new information:

-Metric is updated

-New information is

stored in routing table


Network Discovery§ Exchange of Routing Information

–Router convergence is reached when

-All routing tables in the network contain the same network information

–Routers continue to exchange routing information

-If no new information is found then Convergence is reached


Network Discovery§ Convergence must be reached before a network is

considered completely operable

§ Speed of achieving convergence consists of 2 interdependent categories

-Speed of broadcasting routing information

-Speed of calculating routes


Routing Table Maintenance§ Periodic Updates: RIPv1 & RIPv2

These are time intervals in which a router sends out its entire routing table.


Routing Table Maintenance§ RIP uses 4 timers

-Update timer -Invalid timer -Holddown timer -Flush timer


Routing Table Maintenance§ Bounded Updates: EIGRP§ EIRPG routing updates are

-Partial updates

-Triggered by topology changes

-Bounded

-Non periodic


Routing Table Maintenance§ Triggered Updates

–Conditions in which triggered updates are sent

-Interface changes state

-Route becomes unreachable

-Route is placed in routing table


Routing Table Maintenance§ Random Jitter

Synchronized updatesA condition where multiple routers on multi access LAN segments transmit routing updates at the same time.§Problems with synchronized updates

-Bandwidth consumption-Packet collisions

§Solution to problems withsynchronized updates

- Used of random variable called RIP_JITTER


Routing Loops

§ Routing loops are

A condition in which a packet is continuously transmitted within a series of routers without ever reaching its destination.


Routing Loops§ Routing loops may be caused by:

-Incorrectly configured static routes-Incorrectly configured route redistribution-Slow convergence-Incorrectly configured discard routes

§ Routing loops can create the following issues-Excess use of bandwidth-CPU resources may be strained-Network convergence is degraded-Routing updates may be lost or not processed in a timely manner


Routing Loops

§ Count to InfinityThis is a routing loop whereby packets bounce infinitely around a network.


Routing Loops§ Setting a maximum

§ Distance Vector routing protocols set a specified metric value to indicate infinity

Once a router “counts to infinity” it marks the route as unreachable


Routing Loops§ Preventing loops with holddown timers

-Holddown timers allow a router to not accept any changes to a route for a specified period of time.-Point of using holddown timers§Allows routing updates to propagate through network with the most current information.


Routing Loops§ The Split Horizon Rule is used to prevent routing

loops

§ Split Horizon rule:

A router should not advertise a network through the interface from which the update came.


Routing Loops§ Split horizon with poison

reverse The rule states that once a router learns of an unreachable route through an interface, advertise it as unreachable back through the same interface


Routing Loops§ IP & TTL

–Purpose of the TTL fieldThe TTL field is found in an IP header and is used to prevent packets from endlessly traveling on a network

§ How the TTL field works-TTL field contains a numeric value

The numeric value is decreased by one by every router on the route to the destination.

If numeric value reaches 0 then Packet is discarded.


Routing Protocols Today§ Factors used to determine whether to use RIP or EIGRP

include-Network size-Compatibility between models of routers-Administrative knowledge


Routing Protocols Today

§ RIP§Features of RIP:

-Supports split horizon & split horizon with poison reverse

-Capable of load balancing

-Easy to configure

-Works in a multi vendor router environment


Routing Protocols Today

§ EIGRP§Features of EIGRP:

-Triggered updates-EIGRP hello protocol used to establish neighbor adjacencies-Supports VLSM & route summarization-Use of topology table to maintain all routes-Classless distance vector routing protocol-Cisco proprietary protocol


Summary§ Characteristics of Distance Vector routing

protocols–Periodic updates–RIP routing updates include the entire routing table–Neighbors are defined as routers that share a link and are configured to use the same protocol

§ The network discovery process for D.V. routing protocol

–Directly connected routes are placed in routing table 1st

–If a routing protocol is configured then•Routers will exchange routing information

–Convergence is reached when all network routers have the same network information


Summary§ D.V. routing protocols maintains routing tables by

–RIP sending out periodic updates–RIP using 4 different timers to ensure information is accurate and convergence is achieved in a timely manner–EIGRP sending out triggered updates

§ D.V. routing protocols may be prone to routing loops– routing loops are a condition in which packets continuously traverse a network–Mechanisms used to minimize routing loops include defining maximum hop count, holddown timers, split horizon, route poisoning and triggered updates


Summary§ Conditions that can lead to routing loops include

–Incorrectly configured static routes–Incorrectly configured route redistribution–Slow convergence–Incorrectly configured discard routes

§ How routing loops can impact network performance includes:

–Excess use of bandwidth–CPU resources may be strained–Network convergence is degraded–Routing updates may be lost or not processed


Summary

§ Routing Information Protocol (RIP)A distance vector protocol that has 2 versions

RIPv1 – a classful routing protocolRIPv2 - a classless routing protocol

§ Enhanced Interior Gateway Routing Protocol (EIGRP)

–A distance vector routing protocols that has some features of link state routing protocols–A Cisco proprietary routing protocol


RIP version 1

Chapter 5


Objectives§ Describe the functions, characteristics, and operation

of the RIPv1 protocol.

§ Configure a device for using RIPv1.

§ Verify proper RIPv1 operation.

§ Describe how RIPv1 performs automatic summarization.

§ Configure, verify, and troubleshoot default routes propagated in a routed network implementing RIPv1.

§ Use recommended techniques to solve problems related to RIPv1


RIPv1

§ RIP Characteristics

-A classful, Distance Vector (DV) routing protocol

-Metric = hop count

-Routes with a hop count > 15 are unreachable

-Updates are broadcast every 30 seconds


RIPv1§ RIP Message Format§ RIP header - divided into

3 fields-Command field-Version field -Must be zero

§ Route Entry - composed of 3 fields

-Address family identifier -IP address-Metric


RIPv1

§ RIP Operation–RIP uses 2 message types:

§Request message

-This is sent out on startup by each RIP enabled interface

-Requests all RIP enabled neighbors to send routing table

§Response message

-Message sent to requesting router containing routing table


RIPv1§ IP addresses initially divided

into classes

-Class A

-Class B

-Class C

§ RIP is a classful routing protocol

-Does not send subnet masks in routing updates


RIPv1§ Administrative Distance

–RIP’s default administrative distance is 120


Basic RIPv1 Configuration§ A typical topology suitable for

use by RIPv1 includes:-Three router set up -No PCs attached to LANs

-Use of 5 different IP subnets


Basic RIPv1 Configuration§ Router RIP Command

–To enable RIP enter:-Router rip at the global configuration prompt-Prompt will look like R1(config-router)#


Basic RIPv1 Configuration§ Specifying Networks

–Use the networkcommand to:

-Enable RIP on all interfaces that belong to this network-Advertise this network in RIP updates sent to other routers every 30 seconds


Verification and Troubleshooting

§ Show ip Route

§ To verify and troubleshoot routing

-Use the following

commands:

-show ip route

-show ip protocols

-debug ip rip



§ show ip protocolscommand

-Displays routing protocol configured on router


Verification and Troubleshooting§ Debug ip rip command

-Used to display RIP routing updates as they are happening



§ Passive interface command

-Used to prevent a router from sending updates through an interface

-Example:

Router(config-router)#passive-interface interface-type interface-number


Verification and Troubleshooting§ Passive interfaces


Automatic Summarization Modified Topology§ The original scenario has been

modified such that:Three classful networks are used:

172.30.0.0/16192.168.4.0/24192.168.5.0/24

The 172.30.0.0/16 network is subnetted into three subnets:

172.30.1.0/24172.30.2.0/24172.30.3.0/24

The following devices are part of the 172.30.0.0/16 classful network address:

All interfaces on R1S0/0/0 and Fa0/0 on R2


Automatic Summarization§ Configuration Details

-To remove the RIP routing process use the following command

No router rip

-To check the configuration use the following command

Show run


Automatic Summarization§ Boundary Routers

–RIP automatically summarizes classful networks–Boundary routers summarize RIP subnets from one major network to another.


Automatic SummarizationProcessing RIP Updates§ 2 rules govern RIPv1 updates:

-If a routing update and the interface it’s received on belong to the samenetwork then

The subnet mask of the interface is applied to the network in the routing update

-If a routing update and the interface it’s received on belong to a differentnetwork then

The classful subnet mask of the network is applied to the network in the routing update.


Automatic Summarization§ Sending RIP Updates

–RIP uses automatic summarization to reduce the size of a routing table.


Automatic Summarization§ Advantages of automatic

summarization:

-The size of routing updates is reduced

-Single routes are used to represent multiple routes which results in faster lookup in the routing table.


Automatic Summarization§ Disadvantage of Automatic Summarization:

-Does not support discontiguous networks


Automatic Summarization§Discontiguous

Topologies do not converge with RIPv1

§ A router will only advertise major network addresses out interfaces that do not belong to the advertised route.


Default Route and RIPv1§Modified Topology: Scenario C

§Default routes Packets that are not defined specifically in a routing table will go to the specified interface for the default route

Example: Customer routers use default routes to connect to an ISP router.

Command used to configure a default route isip route 0.0.0.0 0.0.0.0 s0/0/1


Default Route and RIPv1


Default Route and RIPv1§ Propagating the Default Route in RIPv1

§ Default-information originate command -This command is used to specify that the router is to originate default information, by propagating the static default route in RIP update.


Summary

§ RIP characteristics include:

Classful, distance vector routing protocol

Metric is Hop Count

Does not support VLSM or discontiguous subnets

Updates every 30 seconds

§ Rip messages are encapsulated in a UDP segment with source and destination ports of 520


Summary: Commands used by RIP

Command Command’s purpose

Rtr(config)#router rip Enables RIP routing process

Rtr(config-router)#network Associates a network with a RIP routing process

Rtr#debug ip rip used to view real time RIP routing updates

Rtr(config-router)#passive-interface fa0/0 Prevent RIP updates from going out an interface

Rtr(config-router)#default-information originate Used by RIP to propagate default routes

Rtr#show ip protocols Used to display timers used by RIP


VLSM and CIDR

Chapter 6


Objectives§ Compare and contrast classful and classless IP

addressing.

§ Review VLSM and explain the benefits of classless IP addressing.

§ Describe the role of the Classless Inter-Domain Routing (CIDR) standard in making efficient use of scarce IPv4 addresses


Introduction§ Prior to 1981, IP addresses used only the first 8 bits to

specify the network portion of the address

§ In 1981, RFC 791 modified the IPv4 32-bit address to allow for three different classes

§ IP address space was depleting rapidlythe Internet Engineering Task Force (IETF) introduced Classless Inter-Domain Routing (CIDR)

–CIDR uses Variable Length Subnet Masking (VLSM) to help conserve address space.

-VLSM is simply subnetting a subnet


Classful and Classless IP Addressing § Classful IP addressing§ As of January 2007, there are over 433 million hosts on

internet§ Initiatives to conserve IPv4 address space include:

-VLSM & CIDR notation (1993, RFC 1519)-Network Address Translation (1994, RFC 1631)-Private Addressing (1996, RFC 1918)


Classful and Classless IP Addressing

§ The High Order Bits

These are the leftmost bits in a 32 bit address


Classful and Classless IP Addressing§ Classes of IP addresses are identified by the decimal number of

the 1st octetClass A address begin with a 0 bit

Range of class A addresses = 0.0.0.0 to 127.255.255.255

Class B address begin with a 1 bit and a 0 bit

Range of class B addresses = 128.0.0.0 to 191.255.255.255

Class C addresses begin with two 1 bits & a 0 bit

Range of class C addresses = 192.0.0.0 to 223.255.255.255.


Classful and Classless IP Addressing§ The IPv4 Classful Addressing Structure (RFC 790)

An IP address has 2 parts:

-The network portion

Found on the left side of an IP address

-The host portion

Found on the right side of an IP address



§ Purpose of a subnet maskIt is used to determine the network portion of an IP address


Classful and Classless IP Addressing§ Classful Routing Updates

-Recall that classful routing protocols (i.e. RIPv1) do not send subnet masks in their routing updates

The reason is that the Subnet mask isdirectly related to the network address


Classful and Classless IP Addressing§ Classless Inter-domain Routing (CIDR – RFC 1517)

§Advantage of CIDR :-More efficient use of IPv4 address space-Route summarization

§Requires subnet mask to be included in routing update because address class is meaninglessRecall purpose of a subnet mask:

-To determine the network and host portion of an IP address


Classful and Classless IP Addressing§ Classless IP Addressing

§ CIDR & Route Summarization

-Variable Length Subnet Masking (VLSM)

-Allows a subnet to be further sub-netted according to individual needs

-Prefix Aggregation a.k.a. Route Summarization

-CIDR allows for routes to be summarized as a single route


Classful and Classless IP Addressing§ Classless Routing Protocol

§ Characteristics of classless routing protocols:

-Routing updates include the subnet mask

-Supports VLSM

Supports Route Summarization



RoutingProtocol

Routing updatesInclude subnet Mask

Supports VLSM

Ability to sendSupernet routes

Classful No No No

Classless Yes Yes Yes

§ Classless Routing Protocol


VLSM§ Classful routing

-only allows for one subnet mask for all networks

§ VLSM & classless routing-This is the process of subnetting a subnet-More than one subnet mask can be used

-More efficient use of IP addresses as compared to classful IP addressing


VLSM§ VLSM – the process of

sub-netting a subnet to fit your needs

-Example:Subnet 10.1.0.0/16, 8 more bits are borrowed again, to create 256 subnets with a /24 mask.

-Mask allows for 254 host addresses per subnet

-Subnets range from: 10.1.0.0 / 24 to 10.1.255.0 / 24


Classless Inter-Domain Routing (CIDR)§ Route summarization done by CIDR

-Routes are summarized with masks that are less than that of the default classful mask

-Example:

172.16.0.0 / 13 is the summarized route for the 172.16.0.0 / 16 to 172.23.0.0 / 16 classful networks


Classless Inter-Domain Routing (CIDR)§ Steps to calculate a route

summary-List networks in binary format-Count number of left most matching bits to determine summary route’s mask-Copy the matching bits and add zero bits to determine the summarized network address


Summary§ Classful IP addressing§IPv4 addresses have 2 parts:

-Network portion found on left side of an IP address-Host portion found on right side of an IP address

§Class A, B, & C addresses were designed to provide IP addresses for different sized organizations§The class of an IP address is determined by the decimal value found in the 1st octet§IP addresses are running out so the use of Classless Inter Domain Routing (CIDR) and Variable Length Subnet Mask (VLSM) are used to try and conserve address space


Summary§ Classful Routing Updates

–Subnet masks are not sent in routing updates

§ Classless IP addressing–Benefit of classless IP addressing

§Can create additional network addresses using a subnet mask that fits your needs

–Uses Classless Interdomain Routing (CIDR)


Summary§ CIDR§ Uses IP addresses more efficiently through

use of VLSM-VLSM is the process of subnetting a subnet

§ Allows for route summarization-Route summarization is representing multiple contiguous routes with a single route


Summary

§ Classless Routing UpdatesSubnet masks are included in updates


RIPv2

Routing Protocols and Concepts – Chapter 7


Objectives§ Encounter and describe the limitations of RIPv1’s

limitations.

§ Apply the basic Routing Information Protocol Version 2 (RIPv2) configuration commands and evaluate RIPv2 classless routing updates.

§ Analyze router output to see RIPv2 support for VLSM and CIDR

§ Identify RIPv2 verification commands and common RIPv2 issues.

§ Configure, verify, and troubleshoot RIPv2 in “hands-on” labs


Introduction§ Chapter focus

-Difference between RIPv1 & RIPv2§RIPv1

-A classful distance vector routing protocol-Does not support discontiguous subnets-Does not support VLSM-Does not send subnet mask in routing update-Routing updates are broadcast

§RIPv2-A classless distance vector routing protocol that is an enhancement of RIPv1’s features.-Next hop address is included in updates-Routing updates are multicast-The use of authentication is an option


Introduction

§ Similarities between RIPv1 & RIPv2

-Use of timers to prevent routing loops

-Use of split horizon or split horizon with poison reverse

-Use of triggered updates

-Maximum hop count of 15


RIPv1 Limitations§ Lab Topology

§ Scenario:§3 router set up§Topology is discontiguous§There exists a static summary route§Static route information can be injected into routing table updates using redistribution.§Routers 1 & 3 contain VLSM networks


RIPv1 Limitations§ Scenario Continued

§ VLSM

-Recall this is sub netting the subnet

§ Private IP addresses are on LAN links

§ Public IP addresses are used on WAN links

§ Loopback interfaces

-These are virtual interfaces that can be pinged and added to routing table


RIPv1 Limitations§ Null Interfaces

§This is a virtual interface that does not need to be created or configured

-Traffic sent to a null interface is discarded

-Null interfaces do not send or receive traffic

§ Static routes and null interfaces§null interfaces will serve as the exit interface for static route

-Example of configuring a static supernet route with a null interface-R2(config)#ip route 192.168.0.0 255.255.0.0 Null0


RIPv1 Limitations§ Route redistribution

-Redistribution command is way to disseminate a static route from one router to another via a routing protocol

-Example

R2(config-router)#redistribute static


RIPv1 Limitations

§ Verifying and Testing ConnectivityUse the following commands:§ show ip interfaces brief§ ping§ traceroute


RIPv1 Limitations§ RIPv1 – a classful routing protocol

-Subnet mask are not sent in updates

-Summarizes networks at major network boundaries

-if network is discontiguous and RIPv1 configured convergence will not be reached


RIPv1 Limitations§Examining the routing tables

-To examine the contents of routing updates use the

debug ip rip command

-If RIPv1 is configured then

Subnet masks will not be included with the network address


RIPv1 Limitations§ RIPv1 does not support

VLSM

Reason: RIPv1 does not send subnet mask in routing updates

§ RIPv1 does summarize routes to the Classful boundary

Or uses the Subnet mask of the outgoing interface to determine which subnets to advertise


RIPv1 Limitations

§ No CIDR Support§ In the diagram R2 will not

include the static route in its update Reason: Classful routing protocols do not support CIDR routes that are summarized with a smaller mask than the classful subnet mask


Configuring RIPv2§ Comparing RIPv1 & RIPv2 Message Formats

-RIPv2 Message format is similar to RIPv1 but has 2 extensions§1st extension is the subnet mask field§2nd extension is the addition of next hop

address


Configuring RIPv2

§ Enabling and Verifying RIPv2

§ Configuring RIP on a Cisco router

By default it is running RIPv1


Configuring RIPv2

§ Configuring RIPv2 on a Cisco router

-Requires using the version 2 command

-RIPv2 ignores RIPv1 updates

§ To verify RIPv2 is configured use the

show ip protocolscommand


Configuring RIPv2

§ Auto-Summary & RIPv2

§ RIPv2 will automatically summarize routes at major network boundaries and can also summarize routes with a subnet mask that is smaller than the classful subnet mask


§ Disabling Auto-Summary in RIPv2

§ To disable automatic summarization issue the no auto-summarycommand

Configuring RIPv2


Configuring RIPv2

§ Verifying RIPv2 Updates

§ When using RIPv2 with automatic summarization turned off

Each subnet and mask has its own specific entry, along with the exit interface and next-hop address to reach that subnet.

§ To verify information being sent by RIPv2 use the

debug ip rip command


VLSM & CIDR

§ RIPv2 and VLSM

§ Networks using a VLSM IP addressing scheme

Use classless routing protocols (i.e. RIPv2) to disseminate network addresses and their subnet masks


VLSM & CIDR

§ CIDR uses Supernetting

Supernetting is a bunch of contiguous classful networks that is addressed as a single network.


VLSM & CIDR§ To verify that

supernets are being sent and received use the following commands

-Show ip route

-Debug ip rip


Verifying & Troubleshooting RIPv2§ Basic Troubleshooting steps

-Check the status of all links

-Check cabling

-Check IP address & subnet mask configuration

-Remove any unneeded configuration commands

§ Commands used to verify proper operation of RIPv2–Show ip interfaces brief–Show ip protocols–Debug ip rip–Show ip route


Verifying & Troubleshooting RIPv2§ Common RIPv2 Issues

§ When trouble shooting RIPv2 examine the following issues:§Version

Check to make sure you are using version 2§Network statements

Network statements may be incorrectly typed or missing

§Automatic summarization

If summarized routes are not needed then disable automatic summarization


Verifying & Troubleshooting RIPv2§ Reasons why it’s good to authenticate routing information

-Prevent the possibility of accepting invalid routing updates

-Contents of routing updates are encrypted

§ Types of routing protocols that can use authentication

-RIPv2

-EIGRP

-OSPF

-IS-IS

-BGP


Summary

RoutingProtocol

DistanceVector

ClasslessRoutingProtocol

UsesHold-DownTimers

Use ofSplit

Horizon or

Split Horizon

w/Poison

Reverse

MaxHop count= 15

AutoSummary

SupportCIDR

SupportsVLSM

Uses Authen-tication

RIPv1 Yes No Yes Yes Yes Yes No No No

RIPv2 Yes Yes Yes Yes Yes Yes Yes Yes Yes


The Routing Table: A Closer Look

Chapter 8


Objectives§ Describe the various route types found in the routing

table structure

§ Describe the routing table lookup process.

§ Describe routing behavior in routed networks.


Introduction

§ Chapter Focus

-Structure of the routing table

-Lookup process of the routing table

-Classless and classful routing behaviors


Routing Table Structure§ Lab Topology

§ 3 router setup-R1 and R2 share a common 172.16.0.0/16 network with 172.16.0.0/24 subnets.-R2 and R3 are connected by the 192.168.1.0/24 network.-R3 also has a 172.16.4.0/24 subnet, which is disconnected, or discontiguous, from the 172.16.0.0 network that R1 and R2 share.


Routing Table Structure§ Routing table entries come from the following sources

-Directly connected networks-Static routes-Dynamic routing protocols


Routing Table Structure

§ Level 1 Routes

§ As soon as the no shutdown command is issued the route is added to routing table



§ Cisco IP routing table is a hierarchical structure

-The reason for this is to speed up lookup process


Routing Table Structure§ Level 1 Routes

-Have a subnet mask equal to or less than the classful mask of the network address.

§ Level 1 route can function as

-Default route

-Supernet route

-Network route


Routing Table Structure§ Level 1 Routes

-Ultimate Route§Includes either:

-A next-hop addressOR

-An exit interface


§ Parent and Child Routes-A parent route is a level 1 route

-A parent route does not contain any next-hop IP address or exit interface information



Routing Table Structure§ Automatic creation of parent

routes-Occurs any time a subnet is added to the routing table

§ Child routes

-Child routes are level 2 routes

-Child routes are asubnet of a classful network address


Routing Table Structure§ Level 2 child routes contain route source & the network

address of the route§ Level 2 child routes are also considered ultimate

routesReason: they contain the next hop address &/or exit

interface


Routing Table Structure§ Both child routes have the same subnet mask

-This means the parent route maintains the /24 mask


Routing Table Structure§ Diagram illustrates 2 child networks belonging to the

parent route 172.16.0.0 / 24


Routing Table Structure§ In classless networks, child routes do not have to share

the same subnet mask



NetworkType

Parent route’sClassful mask is

Displayed

Term variably

subnetted is seen in parent route in routing

table

Includes the # of different

masks of child routes

Subnet mask included with each

child route entry

Class-ful

No No No No

Class-less

Yes Yes Yes Yes

§ Parent & Child Routes: Classless Networks



§ Parent & Child Routes: Classless Networks


Routing Table Lookup Process§ The Route Lookup Process

§ Examine level 1 routes-If best match a level 1 ultimate route and is not a parent route this route is used to forward packet

§ Router examines level 2 (child) routes-If there is a match with level 2 child route then that subnet is used to forward packet

-If no match thendetermine routing behavior type

§ Router determines classful or classless routing behavior-If classful then packet is dropped

-If classless then router searches level one supernet and default routes-If there exists a level 1 supernet or default route match then Packet is forwarded. If not packet is dropped


Routing Table Lookup Process§ Longest Match: Level 1 Network Routes

–Best match is also known as the longest match –The best match is the one that has the most number of left most bits matching between the destination IP address and the route in the routing table.


Routing Table Lookup Process§ Finding the subnet mask

used to determine the longest match

Scenario:–PC1 pings 192.168.1.2–Router examines level 1 route for best match–There exist a match between192.168.1.2 & 192.168.1.0 / 24–Router forwards packets out s0/0/0


Routing Table Lookup Process§ The process of matching

-1st there must be a match made between the parent route & destination IP

-If a match is made then an attempt at finding a match between the destination IP and the child route is made.


Routing Table Lookup Process§ Finding a match between packet’s destination IP address

and the next route in the routing table

-The figure shows a match between the destination IP of 192.168.1.0 and the level one IP of 192.168.1.0 / 24 then packet forwarded out s0/0/0


Routing Table Lookup Process§ Level 1 Parent & Level 2 Child Routes§ Before level 2 child routes are examined

-There must be a match between classful level one parent route and destination IP address.


Routing Table Lookup Process§ After the match with parent route has been made Level 2 child

routes will be examined for a match

-Route lookup process searches for child routes with a match with destination IP


Routing Table Lookup Process§ How a router finds a match with one of the level 2 child

routes-First router examines parent routes for a match-If a match exists then:

§Child routes are examined§Child route chosen is the one with the longest match


Routing Table Lookup Process

§ Example: Route Lookup Process with VLSM

-The use of VLSM does not change the lookup process

-If there is a match between destination IP address and the level 1 parent route then

-Level 2 child routes will be searched


Routing Behavior§ Classful & classless routing protocols

Influence how routing table is populated§ Classful & classless routing behaviors

Determines how routing table is searched after it is filled


Routing Behavior§ Classful Routing

Behavior: no ip classless

§ What happens if there is not a match with any level 2 child routes of the parent?-Router must determine if the routing behavior is classless or classful

-If router is utilizing classful routing behavior then

-Lookup process is terminated and packet is dropped


Routing Behavior§ Classful Routing Behavior – Search Process

§ An example of when classful routing behavior is in effect and why the router drops the Packet

-The destination’s subnet mask is a /24 and none of the child routes left most bits match the first 24 bits. This means packet is dropped


Routing Behavior

§ Classful Routing Behavior – Search Process

§ The reason why the router will not search beyond the child routes

§Originally networks were all classful§This meant an organization could subnet a major network address and “enlighten” all the organization’s routers about the subnetting§Therefore, if the subnet was not in the routing table, the subnet did not exist and packet was dropped


Routing Behavior

§ ip Classless

§ Beginning with IOS 11.3, ip classless was configured by default

§ Classless routing behavior works for -Discontiguous networks

And-CIDR supernets


Routing Behavior§ Classless Routing Behavior: ip classless

§ Route lookup process when ip classless is in use

-If classless routing behavior in effect then§Search level 1 routes§Supernet routes Checked first

-If a match exists then forward packet§Default routes Checked second

If there is no match or no default route then the

§Packet is dropped


Routing Behavior§ Classless Routing Behavior – Search Process§ Router begins search process by finding a match between

destination IP and parent routeAfter finding the above mentioned match, then there is a search of the child route


Routing Behavior§ Classless Routing Behavior – Search Process

§ If no match is found in child routes of previous slide then

Router continues to search the routing table for a match that may have fewer bits in the match


Routing Behavior

§ Classful vs. Classless Routing Behavior-It is recommended to use classless routing behavior §Reason: so supernet and default routes can

be used whenever needed


SummaryContent/structure of a routing table§ Routing table entries

-Directly connected networks-Static route-Dynamic routing protocols

§ Routing tables are hierarchical-Level 1 route

Have a subnet mask that is less than or equal to classful subnet mask for the network address

-Level 2 routeThese are subnets of a network address


SummaryRouting table lookup process§ Begins with examining level 1 routes for best match with packet’s destination IP

§ If the best match = an ultimate route then-Packet is forwarded -Else--Parent route is examined

If parent route & destination IP match then Level 2 (child) routes are examined

Level 2 route examination§ If a match between destination IP and child route found then

Packet forwarded -Else § If Router is using classful routing behavior then

Packet is dropped -Else

§ If router is using classless routing behavior thenRouter searches Level 1 supernet & default routes for a match

§ If a match is found then Packet if forwarded -Else§ Packet is dropped


Summary§ Routing behaviors

-This refers to how a routing table is searched

§ Classful routing behavior-Indicated by the use of the no ip classless command

-Router will not look beyond child routes for a lesser match

§ Classless routing behavior-Indicated by the use of the ip classless command

-Router will look beyond child routes for a lesser match


EIGRP



Objectives§ Describe the background and history of Enhanced

Interior Gateway Routing Protocol (EIGRP).

§ Examine the basic EIGRP configuration commands and identify their purposes.

§ Calculate the composite metric used by EIGRP.

§ Describe the concepts and operation of DUAL.

§ Describe the uses of additional configuration commands in EIGRP.


Introduction


EIGRP

§ Roots of EIGRP: IGRP-Developed in 1985 to overcome RIPv1’s limited hop count-Distance vector routing protocol-Metrics used by IGRP§bandwidth (used by default)§Delay (used by default)§reliability§load

-Discontinued support starting with IOS 12.2(13)T & 12.2(R1s4)S


EIGRPEIGRP Message Format

§ EIGRP Header§Data link frame header - contains source and destination MAC address§IP packet header - contains source & destination IP address§EIGRP packet header - contains AS number§Type/Length/Field - data portion of EIGRP message


EIGRP

§ EIGRP packet headercontains

–Opcode field–Autonomous System number

§ EIGRP Parameters contains–Weights–Hold time


EIGRP

§ TLV: IP internal contains–Metric field–Subnet mask field–Destination field

§ TLV: IP external contains–Fields used when external routes are imported intoEIGRP routing process


EIGRPProtocol Dependent

Modules (PDM)§ EIGRP uses PDM to route

several different protocols i.e. IP, IPX & AppleTalk

§ PDMs are responsible for the specific routing task for each network layer protocol


EIGRPReliable Transport Protocol (RTP)§ Purpose of RTP

–Used by EIGRP to transmit and receive EIGRP packets

§ Characteristics of RTP–Involves both reliable & unreliable delivery of EIGRP packet§Reliable delivery requires acknowledgment from destination§Unreliable delivery does not require an acknowledgement from destination

–Packets can be sent §Unicast§Multicast

–Using address 224.0.0.10


EIGRPEIGRP’s 5 Packet Types

§ Hello packets–Used to discover & form adjacencies with neighbors


EIGRP

§ Update packets–Used to propagate routing information

§ Acknowledgement packets

–Used to acknowledge receipt of update, query & reply packets


EIGRP§ Query & Reply packets

§Used by DUAL for searching for networks§Query packets

-Can use §Unicast§Multicast

§Reply packet -Use only§unicast


EIGRP§ Purpose of Hello Protocol

–To discover & establish adjacencies with neighbor routers

§ Characteristics of hello protocol–Time interval for sending hello packet§Most networks it is every 5 seconds§Multipoint non broadcast multi-access networks

–Unicast every 60 seconds-Holdtime

§This is the maximum time router should wait before declaring a neighbor down§Default holdtime

–3 times hello interval


EIGRPEIGRP Bounded Updates§ EIGRP only sends update when there is a change in

route status§ Partial update

–A partial update includes only the route information that has changed – the whole routing table is NOT sent

§ Bounded update–When a route changes, only those devices that are impacted will be notified of the change

§ EIGRP’s use of partial bounded updates minimizes use of bandwidth


EIGRP

Diffusing Update Algorithm (DUAL)–Purpose

•EIGRP’s primary method for preventing routing loops–Advantage of using DUAL

•Provides for fast convergence time by keeping a list of loop-free backup routes


EIGRP

§ Administrative Distance (AD)–Defined as the trustworthiness of the source route

§ EIGRP default administrative distances–Summary routes = 5–Internal routes = 90–Imported routes = 170


EIGRP

Authentication§ EIGRP can

– Encrypt routing information– Authenticate routing information


EIGRPNetwork Topology§ Topology used is the same as previous chapters with

the addition of an ISP router


EIGRP

§ EIGRP will automatically summarize routes at classful boundaries


Basic EIGRP Configuration § Autonomous System (AS) & Process IDs

–This is a collection of networks under the control of a single authority (reference RFC 1930)–AS Numbers are assigned by IANA–Entities needing AS numbers§ISP§Internet Backbone prodiers§Institutions connecting to other institutions using AS numbers


Basic EIGRP Configuration§ EIGRP autonomous system

number actually functions as a process ID

§ Process ID represents an instance of the routing protocol running on a router

§ ExampleRouter(config)#router

eigrp autonomous-system


Basic EIGRP ConfigurationThe router eigrp command

§ The global command that enables eigrp isrouter eigrp autonomous-system -All routers in the EIGRP routing domain must use the same process ID number (autonomous-system number)


Basic EIGRP ConfigurationThe Network Command

§ Functions of the network command–Enables interfaces to transmit & receive EIGRP updates–Includes network or subnet in EIGRP updates

§ Example–Router(config-router)#network network-address


Basic EIGRP Configuration§ The network Command with a Wildcard Mask

-This option is used when you want to configure EIGRP to advertise specific subnets-Example

Router(config-router)#network network-address [wildcard-mask]


Basic EIGRP ConfigurationVerifying EIGRP§ EIGRP routers must establish adjacencies with their

neighbors before any updates can be sent or received

§ Command used to view neighbor table and verify that EIGRP has established adjacencies with neighbors is

show ip eigrp neighbors


EIGRP

§ The show ip protocolscommand is also used to verify that EIGRP is enabled


Basic EIGRP Configuration

Examining the Routing Table§ The show ip route

command is also used to verify EIGRP

§ EIGRP routes are denoted in a routing table by the letter “D”

§ By default , EIGRP automatically summarizes routes at major network boundary


Basic EIGRP Configuration§ Introducing the Null0 Summary Route

–Null0 is not a physical interface–In the routing table summary routes are sourced from Null0§Reason: routes are used for advertisement purposes

–EIGRP will automatically include a null0 summary route as child route when 2 conditions are met§At least one subnet is learned via EIGRP§Automatic summarization is enabled


Basic EIGRP Configuration§ R3’s routing table shows

that the 172.16.0.0/16 network is automatically summarized by R1 & R3


EIGRP Metric CalculationEIGRP Composite Metric & the K Values§ EIGRP uses the following values in its composite

metric-Bandwidth, delay, reliability, and load

§ The composite metric used by EIGRP– formula used has values K1 àK5

K1 & K3 = 1all other K values = 0


EIGRP Metric Calculation§ Use the sh ip protocols command to verify the K

values


EIGRP Metric CalculationEIGRP Metrics

§ Use the show interfaces command to view metrics

§ EIGRP MetricsBandwidth – EIGRP uses a static bandwidth to calculate metricMost serial interfaces use a default bandwidth value of 1.544Mbos (T1)


EIGRP Metric CalculationEIGRP Metrics

§ Delay is the defined as the measure of time it takes for a packet to traverse a route

-it is a static value based on link type to which interface is connected


EIGRP Metric Calculation§ Reliability (not a default EIGRP metric)

-A measure of the likelihood that a link will fail-Measure dynamically & expressed as a fraction of 255

the higher the fraction the better the reliability

§ Load (not a default EIGRP metric)– A number that reflects how much traffic is using a link– Number is determined dynamically and is expressed as a fraction of 255§The lower the fraction the less the load on the link


EIGRP Metric CalculationUsing the Bandwidth Command

§ Modifying the interface bandwidth-Use the bandwidth command-Example

Router(config-if)#bandwidth kilobits

§ Verifying bandwidth –Use the show interface command

§ Note – bandwidth command does not change thelink’s physical bandwidth


EIGRP Metric Calculation§ The EIGRP metric can be determined by examining the

bandwidth delay


EIGRP Metric Calculation

§ EIGRP uses the lowest bandwidth (BW)in its metric calculation

Calculated BW = reference BW / lowest BW(kbps)

§ Delay – EIGRP uses the cumulative sum of all outgoing interfaces

Calculated Delay = the sum of outgoing interface delays

§ EIGRP Metric = calculated BW + calculated delay


EIGRP Metric Calculation


DUAL Concepts

§ The Diffusing Update Algorithm (DUAL) is used to prevent looping


DUAL Concepts

§ SuccessorThe best least cost route to a destination found in the routing table

§ Feasible distanceThe lowest calculated metricalong a path to a destination network


DUAL ConceptsFeasible Successors, Feasibility Condition & Reported

Distance

§ Feasible Successor

-This is a loop free backup route to same destination as successor route


DUAL Concepts

§ Reported distance (RD)

-The metric that a router reports to a neighbor about its own cost to that network

Feasible Successors, Feasibility Condition & Reported Distance


DUAL Concepts

§ Feasibility Condition (FC)

-Met when a neighbor’s RD is less thanthe local router’s FD to the same destination network


DUAL Concepts

§ Topology Table: Successor & Feasible Successor

§ EIGRP Topology table–Viewed using the show ip

eigrp topology command§Contents of table include:

– all successor routes– all feasible successor

routes


DUAL Concepts

§ EIGRP Topology Table dissected


DUAL ConceptsTopology Table: No

Feasible Successor§ A feasible successor may

not be present because the feasibility condition may not be met

-In other words, the reported distance of the neighbor is greater than or equal to the current feasible distance


DUAL Concepts§ Finite Sate Machine (FSM)

–An abstract machine that defines a set of possible states something can go through, what event causes those states and what events result form those states–FSMs are used to describe how a device, computer program, or routing algorithm will react to a set of input events


DUAL Concepts

§ DUAL FSM–Selects a best loop-free path to a destination–Selects alternate routes by using information in EIGRP tables


DUAL ConceptsFinite State Machines (FSM)

§ To examine output from EIGRP’s finite state machine us the debug eigrp fsm command


More EIGRP Configurations

The Null0 Summary Route§ By default, EIGRP uses the Null0 interface to discard

any packets that match the parent route but do not match any of the child routes

§ EIGRP automatically includes a null0 summary route as a child route whenever both of the following conditions exist

–One or subnets exists that was learned via EIGRP–Automatic summarization is enabled


More EIGRP ConfigurationsThe Null0 Summary Route



Disabling Automatic Summarization§ The auto-summary command permits EIGRP to

automatically summarize at major network boundaries§ The no auto-summary command is used to disable

automatic summarization–This causes all EIGRP neighbors to send updates that will not be automatically summarized§this will cause changes to appear in both

-routing tables -topology tables



Manual Summarization

§ Manual summarization can include supernetsReason: EIGRP is a classless routing protocol & include subnet mask in update

§ Command used to configure manual summarization –Router(config-if)#ip summary-address eigrp as-number

network-address subnet-mask


More EIGRP Configurations§ Configuring a summary route in EIGRP



EIGRP Default Routes

§ “quad zero” static default route-Can be used with any currently supported routing protocol-Is usually configured on a router that is connected a network outside the EIGRP domain

§ EIGRP & the “Quad zero” static default route–Requires the use of the redistribute static command to disseminate default route in EIGRP updates


More EIGRP ConfigurationsFine-Tuning EIGRP

§ EIGRP bandwidth utilization-By default, EIGRP uses only up to 50% of interface bandwidth for EIGRP information-The command to change the percentage of bandwidth used by EIGRP is

Router(config-if)#ip bandwidth-percent eigrp as-number percent


More EIGRP Configurations§ Configuring Hello Intervals and Hold Times

-Hello intervals and hold times are configurable on a per-interface basis-The command to configure hello interval is

Router(config-if)#ip hello-interval eigrp as-number seconds

§ Changing the hello interval also requires changing the hold time to a value greater than or equal to the hello interval

-The command to configure hold time value isRouter(config-if)#ip hold-time eigrp as-number seconds


Summary§ Background & History

–EIGRP is a derivative of IGRP§EIGRP is a Cisco proprietary distance vector routing protocol released in 1994

§ EIGRP terms and characteristics–EIGPR uses RTP to transmit & receive EIGRP packets–EIGRP has 5 packet type:§Hello packets§Update packets§Acknowledgement packets§Query packets§Reply packets

–Supports VLSM & CIDR


Summary

§ EIGRP terms and characteristics–EIGRP uses a hello protocol§Purpose of hello protocol is to discover & establish adjacencies

–EIGRP routing updates§Aperiodic§Partial and bounded§Fast convergence


Summary

§ EIGRP commands–The following commands are used for EIGRP configuration§RtrA(config)#router eigrp [autonomous-system #]§RtrA(config-router)#network network-number

–The following commands can be used to verify EIGRP§Show ip protocols§Show ip eigrp neighbors§Show ip route


Summary

§ EIGRP metrics include–Bandwidth (default)–Delay (default)–Reliability–Load


Summary§DUAL

–Purpose of DUAL§To prevent routing loops

–Successor§Primary route to a destination

–Feasible successor§Backup route to a destination

–Feasible distance§Lowest calculated metric to a destination

–Reported distance§The distance towards a destination as advertised by an upstream neighbor


Summary§ Choosing the best route

–After router has received all updates from directly connected neighbors, it can calculate its DUAL§1st metric is calculated for each route§2nd route with lowest metric is designated successor & is placed in routing table§3rd feasible successor is found

–Criteria for feasible successor: it must have lower reported distance to the destination than the installed route’s feasible distance–Feasible routes are maintained in topology table


Summary

§ Automatic summarization–On by default–Summarizes routes on classful boundary–Summarization can be disabled using the following command§RtrA(config-if)#no auto-summary


Link-State Routing Protocols



Objectives§ Describe the basic features & concepts of link-state

routing protocols.

§ List the benefits and requirements of link-state routing protocols.


Introduction


Link-State Routing§ Link state routing protocols

-Also known as shortest path first algorithms

-These protocols built around Dijkstra’s SPF


Link-State RoutingDikjstra’s algorithm also known as the shortest path first

(SPF) algorithm


Link-State Routing§ The shortest path to a destination is not necessarily the

path with the least number of hops


Link-State RoutingLink-State Routing Process§ How routers using Link State Routing Protocols reach

convergence-Each routers learns about its own directly connected networks-Link state routers exchange hello packet to “meet” other directly connected link state routers.-Each router builds its own Link State Packet (LSP) which includes information about neighbors such as neighbor ID, link type, & bandwidth.-After the LSP is created the router floods it to all neighbors who then store the information and then forward it until all routers have the same information.-Once all the routers have received all the LSPs, the routers then construct a topological map of the network which is used to determine the best routes to a destination


Link-State Routing

§ Directly Connected Networks§ Link

This is an interface on a router

§ Link stateThis is the information about the state of the links


Link-State RoutingSending Hello Packets to Neighbors§ Link state routing protocols use a hello protocol

Purpose of a hello protocol:-To discover neighbors (that use the same link state routing protocol) on its link


Link-State RoutingSending Hello Packets to

Neighbors§ Connected interfaces that are using

the same link state routing protocols will exchange hello packets.

§ Once routers learn it has neighbors they form an adjacency

-2 adjacent neighbors will exchange hello packets

-These packets will serve as a keep alive function


Link-State RoutingBuilding the Link State Packet

§ Each router builds its own Link State Packet (LSP)

Contents of LSP:

-State of each directly connected link

-Includes information about neighbors such as neighbor ID, link type, & bandwidth.


Link-State RoutingFlooding LSPs to Neighbors

§ Once LSP are created they are forwarded out to neighbors.

-After receiving the LSP the neighbor continues to forward it throughout routing area.


Link-State Routing§ LSPs are sent out under the following conditions

-Initial router start up or routing process-When there is a change in topology


Link-State RoutingConstructing a link state data base

§ Routers use a database to construct a topology map of the network


Link-State Routing


Link-State RoutingShortest Path First (SPF) Tree§ Building a portion of the SPF tree

Process begins by examining R2’s LSP information-R1 ignores 1st LSP

Reason: R1 already knows it’s connected to R2


Link-State Routing§ Building a portion of the SPF tree

-R1 uses 2nd LSPReason: R1 can create a link from R2 to R5. This information is added to R1’s SPF tree


Link-State Routing§ Building a portion of the SPF tree

-R1 uses 3rd LSP§Reason: R1 learns that R2 is connected to

10.5.0.0/16. §This link is added to R1’s SPF tree.


Link-State Routing§ Determining the shortest path

The shortest path to a destination determined by adding the costs & finding the lowest cost


Link-State Routing§ Once the SPF algorithm has determined the shortest

path routes, these routes are placed in the routing table.


Link-State Routing Protocols

Routingprotocol

BuildsTopological

map

Router can independently determine the

shortest path to every network.

ConvergenceA periodic/

event driven routing updates

Useof

LSP

Distance vector

No No Slow Generally No No

Link State Yes Yes Fast Generally Yes Yes

Advantages of a Link-State Routing Protocol


Link-State Routing ProtocolsRequirements for using a link state routing protocol§ Memory requirements

Typically link state routing protocols use more memory

§ Processing RequirementsMore CPU processing is required of link state routing protocols

§ Bandwidth RequirementsInitial startup of link state routing protocols can consume lots of bandwidth


Link-State Routing Protocols§ 2 link state routing protocols used for routing IP

-Open Shortest Path First (OSPF)-Intermediate System-Intermediate System (IS-IS)


Summary

§ Link State Routing protocols are also known as Shortest Path First protocols§ Summarizing the link state process

-Routers 1ST learn of directly connected networks-Routers then say “hello” to neighbors-Routers then build link state packets-Routers then flood LSPs to all neighbors-Routers use LSP database to build a network topology map & calculate the best path to each destination


Summary

§ LinkAn interface on the router

§ Link StateInformation about an interface such as

-IP address-Subnet mask-Type of network-Cost associated with link-Neighboring routers on the link


Summary

§ Link State Packets

After initial flooding, additional LSP are sent out when a change in topology occurs

§ Examples of link state routing protocols

-Open shortest path first

-IS-IS


OSPF

Chapter 11


Objectives§ Describe the background and basic features of OSPF

§ Identify and apply the basic OSPF configuration commands

§ Describe, modify and calculate the metric used by OSPF

§ Describe the Designated Router/Backup Designated Router (DR/BDR) election process in multiaccess networks

§ Describe the uses of additional configuration commands in OSPF


Introduction


Introduction to OSPFBackground of OSPF§ Began in 1987§ 1989 OSPFv1 released in RFC 1131

This version was experimental & never deployed§ 1991 OSPFv2 released in RFC 1247§ 1998 OSPFv2 updated in RFC 2328§ 1999 OSPFv3 published in RFC 2740


Introduction to OSPFOSPF Message Encapsulation§ OSPF packet type

There exist 5 types

§ OSPF packet header Contains - Router ID and area ID and Type code for OSPF packet type

§ IP packet header

Contains - Source IP address, Destination IP address, & Protocol field set to 89


Introduction to OSPFOSPF Message Encapsulation§ Data link frame header

Contains - Source MAC address and Destination MAC address


Introduction to OSPFOSPF Packet Types


Introduction to OSPFHello Protocol§ OSPF Hello Packet

–Purpose of Hello Packet§ Discover OSPF neighbors & establish adjacencies§ Advertise guidelines on which routers must agree to become neighbors§ Used by multi-access networks to elect a designated router and a backup designated router


Introduction to OSPF§ Hello Packets continued

Contents of a Hello Packetrouter ID of transmitting router

§ OSPF Hello Intervals–Usually multicast (224.0.0.5)–Sent every 30 seconds for NBMA segments

§ OSPF Dead Intervals–This is the time that must transpirebefore the neighbor is considered down

–Default time is 4 timesthe hello interval


Introduction to OSPF§ Hello protocol packets contain information that is used

in electing-Designated Router (DR)§ DR is responsible for updating all other OSPF routers-Backup Designated Router (BDR)§ This router takes over DR’s responsibilities if DR fails


Introduction to OSPFOSPF Link-state Updates§ Purpose of a Link State Update (LSU)

Used to deliver link state advertisements§ Purpose of a Link State Advertisement (LSA)

Contains information about neighbors & path costs


Introduction to OSPFOSPF Algorithm

§ OSPF routers build & maintain link-state database containing LSA received from other routers

–Information found in database is utilized upon execution of Dijkstra SPF algorithm–SPF algorithm used to create SPF tree–SPF tree used to populate routing table


Introduction to OSPFAdministrative Distance

§ Default Administrative Distance for OSPF is 110


Introduction to OSPF§ OSPF Authentication

–Purpose is to encrypt & authenticate routing information–This is an interface specific configuration–Routers will only accept routing information from other routers that have been configured with the same password or authentication information


Basic OSPF ConfigurationLab Topology

§ Topology used for this chapter Discontiguous IP addressing schemeSince OSPF is a classless routing protocol the subnet mask is configured in


Basic OSPF ConfigurationThe router ospf command§ To enable OSPF on a router use the following

commandR1(config)#router ospf process-idProcess id § A locally significant number between 1 and 65535

-this means it does not have to match other OSPF routers


Basic OSPF Configuration§ OSPF network command

-Requires entering: network addresswildcard mask - the inverse of the subnet

maskarea-id - area-id refers to the OSPF area.

OSPF area is a group of routers that share link state information

-Example: Router(config-router)#network network-address wildcard-ask area area-id


Basic OSPF Configuration

§ Router ID–This is an IP address used to identify a router–3 criteria for deriving the router ID§Use IP address configured with OSPF router-id command

-Takes precedence over loopback and physical interface addresses

§If router-id command not used then router chooses highest IP address of any loopback interfaces§If no loopback interfaces are configured then the highest IP address on any active interface is used


Basic OSPF ConfigurationOSPF Router ID§ Commands used to verify current router ID

–Show ip protocols–Show ip ospf–Show ip ospf interface



OSPF Router ID§ Router ID & Loopback addresses

-Highest loopback address will be used as router ID if router-id command isn’t used-Advantage of using loopback address

the loopback interface cannot fail à OSPF stability

§ The OSPF router-id command–Introduced in IOS 12.0–Command syntax§Router(config)#router ospfprocess-id§Router(config-router)#router-idip-address

§ Modifying the Router ID–Use the command Router#clear ip ospf process


Basic OSPF ConfigurationVerifying OSPF§ Use the show ip ospf command to verify & trouble

shoot OSPF networksCommand will display the following:§ Neighbor adjacency

-No adjacency indicated by -§Neighboring router’s Router ID is not displayed§A state of full is not displayed

-Consequence of no adjacency-§No link state information exchanged§Inaccurate SPF trees & routing tables



Command Description

Show ip protocols

Displays OSPF process ID, router ID, networks router is advertising & administrative distance

Show ip ospf

Displays OSPF process ID, router ID, OSPF area information & the last time SPF algorithm calculated

Show ip ospf interfaceDisplays hello interval and dead interval

Verifying OSPF - Additional Commands


Basic OSPF ConfigurationExamining the routing table§ Use the show ip route command to display the routing

table-An “O’ at the beginning of a route indicates that the router source is OSPF-Note OSPF does not automatically summarize at major network boundaries


OSPF Metric§ OSPF uses cost as the metric for determining the

best route-The best route will have the lowest cost-Cost is based on bandwidth of an interface§Cost is calculated using the formula

108 / bandwidth-Reference bandwidth§defaults to 100Mbps§can be modified using§auto-cost reference-bandwidth command


OSPF Metric§ COST of an OSPF route

Is the accumulated value from one router to the next


OSPF Metric§ Usually the actual speed of a link is different than the

default bandwidth–This makes it imperative that the bandwidth value reflects link’s actual speed§Reason: so routing table has best path information

§ The show interface command will display interface’s bandwidth

-Most serial link default to 1.544Mbps


Basic OSPF ConfigurationModifying the Cost of a link§ Both sides of a serial link should be configured with the

same bandwidth–Commands used to modify bandwidth value§Bandwidth command

–Example: Router(config-if)#bandwidthbandwidth-kbps§ip ospf cost command – allows you to directly specify interface cost

-Example:R1(config)#interface serial 0/0/0R1(config-if)#ip ospf cost 1562


Basic OSPF ConfigurationModifying the Cost of the link§ Difference between bandwidth command & the ip ospf

cost command–Ip ospf cost command§Sets cost to a specific value

–Bandwidth command§Link cost is calculated


OSPF and Multiaccess NetworksChallenges in Multiaccess Networks§ OSPF defines five network types:

–Point-to-point –Broadcast Multiaccess –Nonbroadcast Multiaccess (NBMA) –Point-to-multipoint –Virtual links


OSPF in Multiaccess Networks§ 2 challenges presented by multiaccess networks

–Multiple adjacencies–Extensive LSA flooding


OSPF in Multiaccess Networks§ Extensive flooding of LSAs

For every LSA sent out there must be an acknowledgement of receipt sent back to transmitting router.

consequence: lots of bandwidth consumed and chaotic traffic


OSPF in Multiaccess Networks§ Solution to LSA flooding issue

is the use of –Designated router (DR)–Backup designated router (BDR)

§ DR & BDR selection–Routers are elected to send & receive LSA

§ Sending & Receiving LSA–DRothers send LSAs via multicast 224.0.0.6 to DR & BDR–DR forward LSA via multicast address 224.0.0.5 to all other routers


OSPF in Multiaccess NetworksDR/BDR Election Process

§ DR/BDR elections DO NOT occur in point to point networks


OSPF in Multiaccess Networks§ DR/BDR elections will take place on multiaccess

networks as shown below


OSPF in Multiaccess Networks

§ Criteria for getting elected DR/BDR1. DR: Router with the highest OSPF

interface priority.2. BDR: Router with the second highest

OSPF interface priority. 3. If OSPF interface priorities are equal, the

highest router ID is used to break the tie.


OSPF in Multiaccess Networks

§ Timing of DR/BDR Election–Occurs as soon as 1st router has its interface enabled on multiaccess network§When a DR is elected it remains as the DR until one of the following occurs

-The DR fails.-The OSPF process on the DR fails.-The multiaccess interface on the DR fails.


OSPF in Multiaccess Networks§ Manipulating the election process

-If you want to influence the election of DR & BDR then do one of the following§Boot up the DR first, followed by the BDR, and then boot all other routers,

OR§Shut down the interface on all routers, followed by a no shutdown on the DR, then the BDR, and then all other routers.


OSPF in Multiaccess NetworksOSPF Interface Priority§ Manipulating the DR/BDR election process continued

–Use the ip ospf priority interface command.–Example:Router(config-if)#ip ospf priority {0 - 255}§Priority number range 0 to 255

–0 means the router cannot become the DR or BDR–1 is the default priority value


More OSPF Configuration

Redistributing an OSPF Default Route

§ Topology includes a link to ISP–Router connected to ISP§Called an autonomous system border router§Used to propagate a default route

–Example of static default routeR1(config)#ip route 0.0.0.0 0.0.0.0 loopback 1

–Requires the use of the default-information originatecommand–Example of default-information originate command

R1(config-router)#default-information originate



Fine-Tuning OSPF

§ Since link speeds are getting faster it may be necessary to change reference bandwidth values

–Do this using the auto-cost reference-bandwidth command–Example:§ R1(config-router)#auto-cost reference-bandwidth 10000



Fine-Tuning OSPF§ Modifying OSPF timers

–Reason to modify timers§Faster detection of network failures

–Manually modifying Hello & Dead intervals§Router(config-if)#ip ospf hello-interval seconds§Router(config-if)#ip ospf dead-interval seconds

–Point to be made§Hello & Dead intervals must be the same between neighbors


Summary§ RFC 2328 describes OSPF link state concepts and

operations

§ OSPF Characteristics–A commonly deployed link state routing protocol–Employs DRs & BDRs on multi-access networks§DRs & BDRs are elected§DR & BDRs are used to transmit and receive LSAs

–Uses 5 packet types:1: HELLO2: DATABASE DESCRIPTION3: LINK STATE REQUEST4: LINK STATE UPDATE 5: LINK STATE ACKNOWLEDGEMENT


Summary§ OSPF Characteristics

–Metric = cost§Lowest cost = best path

§ Configuration–Enable OSPF on a router using the following command§R1(config)#router ospf process-id

–use the network command to define which interfaces will participate in a given OSPF process§Router(config-router)#network network-address

wildcard-mask area area-id


Summary§ Verifying OSPF configuration

–Use the following commands§show ip protocol§show ip route§show ip ospf interface§show ip ospf neighbor

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