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CCNA EXPLORATION V4.0ROUTING PROTOCOLS AND CONCEPTS

ACCESSIBLE INSTRUCTOR MATERIALS POWERPOINT OBJECTIVES

Prepared by

Cisco Learning Institute

June 23, 2008


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June 23, 2008 Page 1

Chapter 1 – Introduction to Routing and Packet Forwarding

Objectives• Identify a router as a computer with an OS and hardware designed for the routing

process.• Demonstrate the ability to configure devices and apply addresses.• Describe the structure of a routing table.• Describe how a router determines a path and switches packets.

Router as a Computer

• Describe the basic purpose of a routero 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

o Routers generally have 2 connections: WAN connection (Connection to ISP) LAN connection

• Data is sent in form of packets between 2 end devices• Routers are used to direct packet to its destination• Routers examine a packet’s destination IP address and determine the best path by

enlisting the aid of a routing table• Router components and their functions

o CPU - Executes operating system instructionso Random access memory (RAM) - Contains the running copy of configuration file.

Stores routing table. RAM contents lost when power is offo Read-only memory (ROM) - Holds diagnostic software used when router is

powered up. Stores the router’s bootstrap program.o Non-volatile RAM (NVRAM) - Stores startup configuration. This may include IP

addresses (Routing protocol, Hostname of router)o Flash memory - Contains the operating system (Cisco IOS)o Interfaces - There exist multiple physical interfaces that are used to connect

network.o Examples of interface types:

Ethernet / fast Ethernet interfaces Serial interfaces Management interfaces

• Router components• Major phases to the router boot-up process

o Test router hardware Power-On Self Test (POST) Execute bootstrap loader


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o Locate & load Cisco IOS software Locate IOS Load IOS

o

Locate & load startup configuration file or enter setup mode Bootstrap program looks for configuration file• Verify the router boot-up process:

o The show version command is used to view information about the router duringthe 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 Interface is a physical connector that enables a router to send or receivepackets

• Each interface connects to a separate network• Consist of socket or jack found on the outside of a router• Types of router interfaces:

o Etherneto Fastetherneto Serialo DSL

o ISDNo Cable

• Two major groups of Router Interfaceso 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

o 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• Routers and the Network Layer

o 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


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o Routers Operate at Layers 1, 2 & 3o Router receives a stream of encoded bitso Bits are decoded and passed to layer 2o

Router de-encapsulates the frameo Remaining packet passed up to layer 3 Routing decision made at this layer by examining destination IP address

o 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:o Topology drawing that Illustrates physical connectivityo Address table that provides the following information:

Device name Interfaces used IP addresses Default gateway

• Basic Router Configuration• A basic router configuration should contain the following:

o Router name - Host name should be uniqueo Banner - At a minimum, banner should warn against unauthorized useo Passwords - Use strong passwordso 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

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

• Verify Basic Router Configurationo Issue the show running-config commando Save the basic router configuration by Issuing the copy running-config startup-

config commando 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:


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o Directly connected networks - this occurs when a device is connected to anotherrouter interface

o Remotely connected networks - this is a network that is not directly connected to

a particular routero Detailed information about the networks include source of information, networkaddress & subnet mask, and Ip address of next-hop router

• Show ip route command is used to view a routing table• Adding a connected network to the routing table

o 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• Static routes in the routing table

o Includes: network address and subnet mask and IP address of next hop router orexit interfaceo Denoted with the code S in the routing tableo 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

o When network only consists of a few routerso Network is connected to internet only through one ISPo Hub & spoke topology is used on a large network

• Connected and Static routes• Dynamic routing protocols

o Used to add remote networks to a routing tableo Are used to discover networkso Are used to update and maintain routing tables

• Automatic network discoveryo Routers are able discover new networks by sharing routing table information

• Maintaining routing tableso 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:

o RIPo IGRP

o EIGRPo OSPF

• Routing Table Principleso 3 principles regarding routing tables:

Every router makes its decisions alone, based on the information it has in itsrouting 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


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• Effects of the 3 Routing Table Principleso Packets are forwarded through the network from one router to another, on a hop

by hop basiso

Packets can take path “X” to a destination but return via path “Y” (Asymmetricrouting)

Router Paths and Packet Switching

• Internet Protocol (IP) packet format contains fields that provide information about thepacket and the sending and receiving hosts

• Fields that are importance for CCNA students:o Destination IP addresso Source IP addresso Version & TTLo IP header length

o Precedence & type of serviceo Packet length

• MAC Layer Frame Format• MAC Frames are also divided into fields. They include:

o Preambleo Start of frame delimitero Destination MAC addresso Source MAC addresso Type/lengtho Data and pado Frame check sequence

• A Metric is a numerical value used by routing protocols help determine the best pathto a destinationo The smaller the metric value the better the path

• 2 types of metrics used by routing protocols are:o Hop count - this is the number of routers a packet must travel through to get to its

destinationo Bandwidth - this is the “speed” of a link also known as the data capacity of a link

• Equal cost metric is a condition where a router has multiple paths to the samedestination that all have the same metric

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

• Path determination is a process used by a router to pick the best path to adestination

• One of 3 path determinations results from searching for the best patho Directly connected networko Remote networko No route determined

• Switching Function of Router is the process used by a router to switch a packet froman incoming interface to an outgoing interface on the same router


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o 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

• As a packet travels from one networking device to anothero The Source and Destination IP addresses NEVER changeo The Source & Destination MAC addresses CHANGE as packet is forwarded from

one router to the nexto 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)• Path determination and switching function details. PC1 Wants to send something to

PC 2 here is part of what happenso

Step 1 - PC1 encapsulates packet into a frame. Frame contains R1’s destinationMAC addresso Step 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

•

Path determination and switching function details. PC1 Wants to send something toPC 2 here is part of what happenso Step 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• Path determination and switching function details. PC1 Wants to send something to

PC 2 here is part of what happenso Step 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


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After finding destination IP in routing table, R3 is directly connected todestination via its fast Ethernet interface

R3 re-encapsulates IP packet with a new Ethernet frame R3 forwards Ethernet packet out Fa0/0 interfaceo 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:

o Hardware i.e. CPU, Memory, System bus, Interfaceso Software used to direct the routing process

IOS Configuration file

•

Routers need to be configured. Basic configuration consists of:o Router nameo Router bannero Password(s)o Interface configurations i.e. IP address and subnet mask

• Routing tables contain the following informationo Directly connected networkso Remotely connected networkso Network addresses and subnet maskso IP address of next hop address

• Routers determine a packets path to its destination by doing the following

o Receiving an encapsulated frame & examining destination MAC address.o If the MAC address matches then Frame is de-encapsulated so that router can

examine the destination IP address.o 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 withappropriate layer 2 frame and send it out to next destination.

o Process continues until packet reaches destination.o Note - only the MAC addresses will change the source and destination IP

addresses do not change.

Chapter 2 – 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.


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• 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 Routero Best Path Selectionso Forwarding packets to destination

• Introducing the Topologyo 3 1800 series routers connected via WAN linkso Each router connected to a LAN represented by a switch and a PC

• Connections of a Router for WANo A router has a DB-60 port that can support 5 different cabling standards

• Connections of a Router for Ethernet

o 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 Interfaceso Show IP router command – used to view routing tableo Show Interfaces command – used to show status of an interfaceo Show IP Interface brief command – used to show a portion of the interface

informationo Show running-config command – used to show configuration file in RAM

• Configuring an Ethernet interfaceo By default all serial and Ethernet interfaces are downo To enable an interface use the No Shutdown command

• Verifying Ethernet interfaceo Show interfaces for fastEthernet 0/0 – command used to show status of fast

Ethernet porto Show ip interface briefo Show running-config

Ethernet interfaces participate in ARP

• Configuring a Serial interfaceo Enter interface configuration modeo Enter in the ip address and subnet masko Enter in the no shutdown command

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


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• Examining Router Interfaceso Physically connecting a WAN Interface.o A WAN Physical Layer connection has sides:

Data Circuit-terminating Equipment (DCE) – This is the service providerCSU/DSU is a DCE device. Data Terminal Equipment (DTE) – Typically the router is the DTE device

• Configuring serial links in a lab environmento One side of a serial connection must be considered a DCEo This requires placing a clocking signal – use the clock rate command.o Example:

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

o Serial Interfaces require a clock signal to control the timing of thecommunications

Routing Table and CDP Protocol

• Purpose of the debug ip routing commando Allows you to view changes that the router performs when adding or removing

routeso Example:

R2#debug ip routing IP routing debugging is on

• To configure an Ethernet interfaceo 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

• When a router only has its interfaces configured & no other routing protocols areconfigured then:o The routing table contains only the directly connected networkso Only devices on the directly connected networks are reachable

• Checking each route in turno The ping command is used to check end to end connectivity

• Purpose of CDPo A layer 2 cisco proprietary tool used to gather information about other directly

connected Cisco devices

• Concept of neighborso 2 types of neighbors

Layer 3 neighbors Layer 2 neighbors

• CDP show commandso Show cdp neighbors command

Displays the following information:- Neighbor device ID


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- Local interface- Holdtime value, in seconds- Neighbor device capability code-

Neighbor hardware platform- Neighbor remote port ID

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

• Disabling CDPo To disable CDP globally use the following command

Router(config)#no cdp run

Static Routes with Exit Interfaces

• Purpose of a static routeo A manually configured route used when routing from a network to a stub network

•

IP route commando To configure a static route use the following command: ip routeo Example:

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

• Dissecting static route syntaxo ip route - Static route commando 172.16.1.0 – Destination network addresso 255.255.255.0 - Subnet mask of destination networko 172.16.2.2 - Serial 0/0/0 interface IP address on R2, which is the "next-hop" to

this network•

Configuring routes to 2 or more remote networkso 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

• Zinin’s 3 routing principleso Principle 1: "Every router makes its decision alone, based on the information it

has in its own routing table.“o Principle 2: "The fact that one router has certain information in its routing table

does not mean that other routers have the same information.“o Principle 3: "Routing information about a path from one network to another does

not provide routing information about the reverse, or return path."

• Using Zinin’s 3 routing principles, how would you answer the following?o Would packets from PC1 reach their destination?

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

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

network.


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• Default Static Routeo 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

o Similar to configuring a static route. Except that destination IP address andsubnet mask are all zeros

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

• Static routes and subnet maskso The routing table lookup process will use the most specific match when

comparing destination IP address and subnet mask• Default static routes and subnet masks

o Since the subnet mask used on a default static route is 0.0.0.0 all packets willmatch• Packet forwarding with static routes. (recall Zinin’s 3 routing principles)• Router 1

o Packet arrives on R1’s Fastethernet 0/0 interface R1 does not have a route tothe destination network, 192.168.2.0/24 R1 uses the default static route

• Packet forwarding with static routes. (recall Zinin’s 3 routing principles)• Router 2

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

• Packet forwarding with static routes. (recall Zinin’s 3 routing principles)

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

• Troubleshooting a Missing Route• Tools that can be used to isolate routing problems include:

o Ping– tests end to end connectivityo Traceroute– used to discover all of the hops (routers) along the path between 2

pointso Show IP route– used to display routing table & ascertain forwarding processo Show ip interface brief- used to show status of router interfaceso Show cdp neighbors detail– used to gather configuration information about

directly connected neighbors• Solving a Missing Route• Finding a missing or mis-configured route requires methodically using the correct

toolso 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


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o If there is a problem with a mis-configured static route remove the static routethen reconfigure the new static route

• Solving a Missing Route

Summary

• Routerso Operate at layer 3o Functions include best path selection & forwarding packets

• Connecting Networkso WANs

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

o LANs Straight through cables or cross over cables are used to connect to

fastethernet port. (The type of cable used depends on what devices arebeing connected).• Cisco Discovery Protocol

o A layer 2 proprietary protocolo Used to discover information about directly connected Cisco devices

• Static Routeso This is a manually configured path that specifies how the router will get to a

certain point using a certain path.• Summary static routes

o This is several static routes that have been condensed into a single static route.• Default route

o It is the route packets use if there is no other possible match for their destinationin the routing table.

• Forwarding of packets when static route is usedo Zinin’s 3 routing principles describe how packets are forwarded.

• Troubleshooting static routes may require some of the following commands:o Pingo Tracerouteo Show IP routeo Show ip interface briefo Show cdp neighbors detail

Chapter 3 – Introduction to Dynamic Routing Protocol

Objectives

• Describe the role of dynamic routing protocols and place these protocols in thecontext of modern network design.

• Identify several ways to classify routing protocols.


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• Describe how metrics are used by routing protocols and identify the metric typesused 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:o Dynamically share information between routers.o Automatically update routing table when topology changes.o Determine best path to a destination

• The purpose of a dynamic routing protocol is to:o Discover remote networkso Maintaining up-to-date routing information

o Choosing the best path to destination networkso Ability to find a new best path if the current path is no longer available

• Components of a routing protocolo Algorithm

In the case of a routing protocol algorithms are used for facilitating routinginformation and best path determination

o Routing protocol messages These are messages for discovering neighbors and exchange of routing

information• Advantages of static routing

o It can backup multiple interfaces/networks on a router

o Easy to configureo No extra resources are neededo More secure

• Disadvantages of static routingo Network changes require manual reconfigurationo Does not scale well in large topologies

Classifying Routing Protocols

• Dynamic routing protocols are grouped according to characteristics. Examplesinclude:o RIP

o IGRPo EIGRPo OSPFo IS-ISo BGP

• Autonomous System is a group of routers under the control of a single authority• Types of routing protocols:

o Interior Gateway Protocols (IGP)


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o Exterior Gateway Protocols (EGP)• Interior Gateway Routing Protocols (IGP)

o Used for routing inside an autonomous system & used to route within the

individual networks themselves.o Examples: RIP, EIGRP, OSPF

• Exterior Routing Protocols (EGP)o Used for routing between autonomous systemso Example: BGPv4

• IGP: Comparison of Distance Vector & Link State Routing Protocolso Distance vector

routes are advertised as vectors of distance & direction incomplete view of network topology Generally, periodic updates

o Link state complete view of network topology is created updates are not periodic

• Classful routing protocolso Do NOT send subnet mask in routing updates

• Classless routing protocolso Do send subnet mask in routing updates

• Convergence is defined as when all routers’ routing tables are at a state ofconsistency

Routing Protocols Metrics

• Metric

o A value used by a routing protocol to determine which routes are better thanothers

• Metrics used in IP routing protocolso Bandwidtho Costo Delayo Hop counto Loado Reliability

• The Metric Field in the Routing Table• Metric used for each routing protocol

o RIP - hop counto IGRP & EIGRP - Bandwidth (used by default), Delay (used by default), Load,

Reliabilityo IS-IS & OSPF – Cost, Bandwidth (Cisco’s implementation)

• Load balancingo This is the ability of a router to distribute packets among multiple same cost

paths


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Administrat ive Distance of a Route

• Purpose of a metrico It’s a calculated value used to determine the best path to a destination

•

Purpose of Administrative Distanceo It’s a numeric value that specifies the preference of a particular route

• Identifying the Administrative Distance (AD) in a routing tableo It is the first number in the brackets in the routing table

• Dynamic Routing Protocols• Directly connected routes

o Have a default AD of 0 • Static Routes

o Administrative distance of a static route has a default value of 1• Directly connected routes

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

Summary

• Dynamic routing protocols fulfill the following functionso Dynamically share information between routerso Automatically update routing table when topology changeso Determine best path to a destination

• Routing protocols are grouped as eithero Interior gateway protocols (IGP) Oro Exterior gateway protocols(EGP)

• Types of IGPs includeo Classless routing protocols - these protocols include subnet mask in routing

updateso Classful routing protocols - these protocols do not include subnet mask in routing

update• 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:

o Route sourceo Administrative distanceo Metric

Chapter 4 – Distance Vector Routing Protocols

Objectives

• Identify the characteristics of distance vector routing protocols.


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• Describe the network discovery process of distance vector routing protocols usingRouting 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:o Routing Information Protocol (RIP)o Interior Gateway Routing Protocol (IGRP)o Enhanced Interior Gateway Routing Protocol (EIGRP)

• Distance Vector Technology

o 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

• Characteristics of Distance Vector routing protocols:o Periodic updateso Neighborso Broadcast updateso Entire routing table is included with routing update

• Routing Protocol Algorithm:o Defined as a procedure for accomplishing a certain task

• Routing Protocol Characteristicso Criteria used to compare routing protocols includes

Time to convergence Scalability Resource usage Implementation & maintenance

Network Discovery

• Router initial start up (Cold Starts)o Initial network discovery

Directly connected networks are initially placed in routing table

• Initial Exchange of Routing Informationo If a routing protocol is configured then

Routers will exchange routing information• Routing updates received from other routers

o Router checks update for new information If there is new information:

- Metric is updated- New information is stored in routing table


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• Exchange of Routing Informationo Router convergence is reached when

All routing tables in the network contain the same network informationo

Routers continue to exchange routing information If no new information is found then Convergence is reached• Convergence must be reached before a network is considered completely operable• Speed of achieving convergence consists of 2 interdependent categories

o Speed of broadcasting routing informationo Speed of calculating routes

Routing Table Maintenance

• Periodic Updates: RIPv1 & RIPv2o These are time intervals in which a router sends out its entire routing table

• RIP uses 4 timers

o Update timero Invalid timero Holddown timero Flush timer

• Bounded Updates: EIGRP• EIRPG routing updates are

o Partial updateso Triggered by topology changeso Boundedo Non periodic

• Triggered Updates

o Conditions in which triggered updates are sent Interface changes state Route becomes unreachable Route is placed in routing table

• Random Jittero Synchronized updates

A condition where multiple routers on multi access LAN segments transmitrouting updates at the same time

Problems with synchronized updates- Bandwidth consumption- Packet collisions

Solution to problems with synchronized updates- Used of random variable called RIP_JITTER

Routing Loops

• Routing loops areo A condition in which a packet is continuously transmitted within a series of

routers without ever reaching its destination.• Routing loops may be caused by:


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o Incorrectly configured static routeso Incorrectly configured route redistributiono Slow convergenceo

Incorrectly configured discard routes• Routing loops can create the following issues

o Excess use of bandwidtho CPU resources may be strainedo Network convergence is degradedo Routing updates may be lost or not processed in a timely manner.

• Count to Infinityo This is a routing loop whereby packets bounce infinitely around a network.

• Setting a maximum• Distance Vector routing protocols set a specified metric value to indicate infinity.

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

• Preventing loops with holddown timerso Holddown timers allow a router to not accept any changes to a route for a

specified period of time.o Point of using holddown timers

Allows routing updates to propagate through network with the most currentinformation.

• The Split Horizon Rule is used to prevent routing loops• Split Horizon rule:

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

• Split horizon with poison reverse

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

• IP & TTLo Purpose of the TTL field

The TTL field is found in an IP header and is used to prevent packets fromendlessly traveling on a network.

o 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 thedestination. If numeric value reaches 0 then Packet is discarded.

Routing Protocols Today

• Factors used to determine whether to use RIP or EIGRP includeo Network sizeo Compatibility between models of routerso Administrative knowledge

• RIPo Features of RIP:


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Supports split horizon & split horizon with poison reverse Capable of load balancing Easy to configure Works in a multi vendor router environment

• EIGRPo 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 protocolso Periodic updateso RIP routing updates include the entire routing tableo 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

o Directly connected routes are placed in routing table 1sto If a routing protocol is configured then

Routers will exchange routing informationo Convergence is reached when all network routers have the same network

information•

D.V. routing protocols maintains routing tables byo RIP sending out periodic updateso RIP using 4 different timers to ensure information is accurate and convergence

is achieved in a timely mannero EIGRP sending out triggered updates

• D.V. routing protocols may be prone to routing loopso Routing loops are a condition in which packets continuously traverse a networko Mechanisms used to minimize routing loops include defining maximum hop

count, holddown timers, split horizon, route poisoning and triggered updates• Conditions that can lead to routing loops include

o Incorrectly configured static routes

o Incorrectly configured route redistributiono Slow convergenceo Incorrectly configured discard routes

• How routing loops can impact network performance includes:o Excess use of bandwidtho CPU resources may be strainedo Network convergence is degradedo Routing updates may be lost or not processed


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

RIPv1 – a classful routing protocol RIPv2 - a classless routing protocol

o 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

Chapter 5 – RIP version 1

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 Characteristicso A classful, Distance Vector (DV) routing protocolo Metric = hop counto Routes with a hop count > 15 are unreachableo Updates are broadcast every 30 seconds

• RIP Message Format• RIP header - divided into 3 fields

o Command fieldo Version fieldo Must be zero

• Route Entry - composed of 3 fieldso Address family identifiero IP addresso Metric RIP Operation

• RIP uses 2 message types:o Request message

This is sent out on startup by each RIP enabled interface Requests all RIP enabled neighbors to send routing table

o Response message Message sent to requesting router containing routing table


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• IP addresses initially divided into classeso Class Ao Class Bo

Class C• RIP is a classful routing protocol

o Does not send subnet masks in routing updates• Administrative Distance

o RIP’s default administrative distance is 120

Basic RIPv1 Configuration

• A typical topology suitable for use by RIPv1 includes:o Three router set upo No PCs attached to LANso Use of 5 different IP subnets

• Router RIP Commando To enable RIP enter:

Router rip at the global configuration prompt Prompt will look like R1(config-router)#

• Specifying Networkso Use the network command 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 routingo Use the following commands:

show ip route show ip protocols debug ip rip

• show ip protocols commando Displays routing protocol configured on router

• Debug ip rip command o Used to display RIP routing updates as they are happening

• Passive interface command o Used to prevent a router from sending updates through an interface

o Example: Router(config-router)#passive-interface interface-type interface-number

• Passive interfaces

Automat ic Summarization

• Modified Topologyo The original scenario has been modified such that:

Three classful networks are used:


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- 172.30.0.0/16- 192.168.4.0/24- 192.168.5.0/24

The 172.30.0.0/16 network is subnetted into three subnets:

- 172.30.1.0/24- 172.30.2.0/24- 172.30.3.0/24

The following devices are part of the 172.30.0.0/16 classful network address:- All interfaces on R1- S0/0/0 and Fa0/0 on R2

• Configuration Detailso To remove the RIP routing process use the following command

No router ripo To check the configuration use the following command

Show run

• Boundary Routerso RIP automatically summarizes classful networkso Boundary routers summarize RIP subnets from one major network to another

• Processing RIP Updateso 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 different

network then- The classful subnet mask of the network is applied to the network in the

routing update.• Sending RIP Updates

o RIP uses automatic summarization to reduce the size of a routing table.• Advantages of automatic summarization:

o The size of routing updates is reducedo Single routes are used to represent multiple routes which results in faster

lookup in the routing table.• Disadvantage of Automatic Summarization:

o Does not support discontiguous networks

• 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


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o Packets that are not defined specifically in a routing table will go to the specifiedinterface for the default route

o Example: Customer routers use default routes to connect to an ISP router. Command used to configure a default route is ip route 0.0.0.0 0.0.0.0 s0/0/1

• Propagating the Default Route in RIPv1• Default-information originate command

o This command is used to specify that the router is to originate defaultinformation, by propagating the static default route in RIP update.

Summary

• RIP characteristics include:o Classful, distance vector routing protocolo Metric is Hop Counto Does not support VLSM or discontiguous subnets

o Updates every 30 seconds• Rip messages are encapsulated in a UDP segment with source and destination

ports of 520• Commands Used by RIP

o Rtr(config)#router rip Enables RIP routing process

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

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

o Rtr(config-router)#passive-interface fa0/0

Prevent RIP updates from going out an interfaceo Rtr(config-router)#default-information originate

Used by RIP to propagate default routeso Rtr#show ip protocols

Used to display timers used by RIP

Chapter 6 – VLSM and CIDR

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 ofthe address


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• In 1981, RFC 791 modified the IPv4 32-bit address to allow for three differentclasses

• IP address space was depleting rapidly•

The Internet Engineering Task Force (IETF) introduced Classless Inter-DomainRouting (CIDR)o 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:

o VLSM & CIDR notation (1993, RFC 1519)

o Network Address Translation (1994, RFC 1631)o Private Addressing (1996, RFC 1918)

• The High Order Bitso These are the leftmost bits in a 32 bit address

• Classes of IP addresses are identified by the decimal number of the 1st octeto Class A address begin with a 0 bit

Range of class A addresses = 0.0.0.0 to 127.255.255.255o Class B address begin with a 1 bit and a 0 bit

Range of class B addresses = 128.0.0.0 to 191.255.255.255o 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.

• The IPv4 Classful Addressing Structure (RFC 790)o 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 masko It is used to determine the network portion of an IP address

• Classful Routing Updateso Recall that classful routing protocols (i.e. RIPv1) do not send subnet masks in

their routing updates.

The reason is that the Subnet mask is directly related to the network address• Classless Inter-domain Routing (CIDR – RFC 1517)

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

o Requires subnet mask to be included in routing update because address class ismeaningless Recall purpose of a subnet mask:


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- To determine the network and host portion of an IP address• Classless IP Addressing• CIDR & Route Summarization

o Variable Length Subnet Masking (VLSM)o Allows a subnet to be further sub-netted according to individual needso Prefix Aggregation a.k.a. Route Summarizationo CIDR allows for routes to be summarized as a single route

• Classless Routing Protocol• Characteristics of classless routing protocols:

o Routing updates include the subnet masko Supports VLSMo Supports Route Summarization

Routing Protocol Routing Updates

Include SubnetMask

Supports VLSM Abili ty to Send

Supernet Routes

Classful No No NoClassless Yes Yes Yes

VLSM

• Classful routingo Only allows for one subnet mask for all networks

• VLSM & classless routingo This is the process of subnetting a subneto More than one subnet mask can be used

o More efficient use of IP addresses as compared to classful IP addressing• VLSM – the process of sub-netting a subnet to fit your needs

o 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 CIDRRoutes are summarized with masks that are less than that of the default classful mask

o 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• Steps to calculate a route summary

o List networks in binary formato Count number of left most matching bits to determine summary route’s masko Copy the matching bits and add zero bits to determine the summarized network

address


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Summary

• Classful IP addressingo

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

o Class A, B, & C addresses were designed to provide IP addresses for differentsized organizations

o The class of an IP address is determined by the decimal value found in the 1stoctet

o 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 conserveaddress space

o Classful Routing Updates Subnet masks are not sent in routing updateso Classless IP addressing Benefit of classless IP addressing

- Can create additional network addresses using a subnet mask that fitsyour needs

Uses Classless Interdomain Routing (CIDR)• CIDR

o Uses IP addresses more efficiently through use of VLSM VLSM is the process of subnetting a subnet

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

route• Classless Routing Updates

o Subnet masks are included in updates

Chapter 7 – RIPv2

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 focuso Difference between RIPv1 & RIPv2

RIPv1


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- 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

o 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 Topologyo 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 networkso 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• Null Interfaces

o 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

o 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

• Route redistributiono Redistribution command is way to disseminate a static route from one router to

another via a routing protocol


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o Example R2(config-router)#redistribute static

• Verifying and Testing Connectivityo

Use the following commands: show ip interfaces brief ping traceroute

• RIPv1 – a classful routing protocolo Subnet mask are not sent in updateso Summarizes networks at major network boundarieso if network is discontiguous and RIPv1 configured convergence will not be

reached• Examining the routing tables

o To examine the contents of routing updates use the debug ip rip command

o If RIPv1 is configured then Subnet masks will not be included with the network address

• RIPv1 does not support VLSMo Reason: RIPv1 does not send subnet mask in routing updates

• RIPv1 does summarize routes to the Classful boundaryo Or uses the Subnet mask of the outgoing interface to determine which subnets to

advertise• No CIDR Support• In the diagram R2 will not include the static route in its update

o Reason: Classful routing protocols do not support CIDR routes that aresummarized with a smaller mask than the classful subnet mask

Configuring RIPv2

• Comparing RIPv1 & RIPv2 Message Formatso 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

• Enabling and Verifying RIPv2• Configuring RIP on a Cisco router

o By default it is running RIPv1 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 protocols command• 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 subnetmask

• Disabling Auto-Summary in RIPv2• To disable automatic summarization issue the no auto-summary command


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• Verifying RIPv2 Updates• When using RIPv2 with automatic summarization turned off

o 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

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

• CIDR uses Supernettingo Supernetting is a bunch of contiguous classful networks that is addressed as a

single network.

o To verify that supernets are being sent and received use the following commands Show ip route Debug ip rip

Verifying & Troubleshooting RIPv2

• Basic Troubleshooting stepso Check the status of all linkso Check cablingo Check IP address & subnet mask configurationo Remove any unneeded configuration commands

• Commands used to verify proper operation of RIPv2

o Show ip interfaces briefo Show ip protocolso Debug ip ripo Show ip route

• Common RIPv2 Issues• When trouble shooting RIPv2 examine the following issues:

o Version Check to make sure you are using version 2

o Network statements Network statements may be incorrectly typed or missing

o Automatic summarization

If summarized routes are not needed then disable automatic summarization• Reasons why it’s good to authenticate routing information

o Prevent the possibility of accepting invalid routing updateso Contents of routing updates are encrypted

• Types of routing protocols that can use authenticationo RIPv2o EIGRPo OSPF


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o IS-ISo BGP

Summary

RoutingProtocol

DistanceVector

Classless RoutingProtocol

Uses Hold- DownTimers

Use of SplitHorizon orSplit Horizonw/ PoisonReverse

RIPv1 Yes No Yes Yes

RIPv2 Yes Yes Yes Yes

RoutingProtocol

Max HopCount =15

AutoSummary

SupportsCIDR

SupportsVLSM

Uses Authentication

RIPv1 Yes Yes No No NoRIPv2 Yes Yes Yes Yes Yes

Chapter 8 – The Routing Table: A Closer Look

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 o Structure of the routing tableo Lookup process of the routing tableo Classless and classful routing behaviors


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Routing Table Structure

• Lab Topology• 3 router setup

o R1 and R2 share a common 172.16.0.0/16 network with 172.16.0.0/24 subnets.o R2 and R3 are connected by the 192.168.1.0/24 network.o 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 entries come from the following sources

o Directly connected networkso Static routeso Dynamic routing protocols

• 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

o The reason for this is to speed up lookup process • 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

o Default routeo Supernet routeo Network route

• Level 1 Routeso Ultimate Route

Includes either:- A next-hop address

OR - An exit interface

• Parent and Child Routeso A parent route is a level 1 routeo A parent route does not contain any next-hop IP address or exit interface

information• Automatic creation of parent routes

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

- Child routes are level 2 routes- Child routes are a subnet of a classful network address

• Level 2 child routes contain route source & the network address of the route• Level 2 child routes are also considered ultimate routes

o Reason: they contain the next hop address &/or exit interface • Both child routes have the same subnet mask

o This means the parent route maintains the /24 mask• Diagram illustrates 2 child networks belonging to the parent route 172.16.0.0 / 24• In classless networks, child routes do not have to share the same subnet mask


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NetworkType

Parent Route’sClassful Mask isDisplayed

Term VariablySubnetted isseen in Parent

Route in RoutingTable

Includes the #of DifferentMasks of Child

Routes

Subnet MaskIncluded witheach Child

Route Entry

Classful No No No NoClassless Yes Yes Yes Yes

• Parent & Child Routes: Classless Networks

Routing Table Lookup Process

• The Route Lookup Processo Examine level 1 routes

If best match a level 1 ultimate route and is not a parent route this route is

used to forward packeto Router examines level 2 (child) routes

If there is a match with level 2 child route then that subnet is used to forwardpacket

If no match then determine routing behavior typeo 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• Longest Match: Level 1 Network Routes

o Best match is also known as the longest matcho 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.• Finding the subnet mask used to determine the longest match

o 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

• The process of matchingo 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 destinationIP and the child route is made.

• Finding a match between packet’s destination IP address and the next route in therouting tableo 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• Level 1 Parent & Level 2 Child Routes• Before level 2 child routes are examined


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o There must be a match between classful level one parent route and destinationIP address.

• After the match with parent route has been made Level 2 child routes will be

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

• How a router finds a match with one of the level 2 child routeso First router examines parent routes for a matcho If a match exists then:

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

• Example: Route Lookup Process with VLSMo The use of VLSM does not change the lookup processo If there is a match between destination IP address and the level 1 parent route

then

o Level 2 child routes will be searched

Routing Behavior

• Classful & classless routing protocolso Influence how routing table is populated

• Classful & classless routing behaviorso Determines how routing table is searched after it is filled

• Classful Routing Behavior: no ip classless• What happens if there is not a match with any level 2 child routes of the parent?

o Router must determine if the routing behavior is classless or classfulo If router is utilizing classful routing behavior then

Lookup process is terminated and packet is dropped• Classful Routing Behavior – Search Process• An example of when classful routing behavior is in effect and why the router drops

the Packeto 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• Classful Routing Behavior – Search Process• The reason why the router will not search beyond the child routes

o Originally networks were all classfulo This meant an organization could subnet a major network address and

“enlighten” all the organization’s routers about the subnetting

o Therefore, if the subnet was not in the routing table, the subnet did not exist andpacket was dropped

• ip Classless• Beginning with IOS 11.3, ip classless was configured by default• Classless routing behavior works for

o Discontiguous networks, ANDo CIDR supernets

• Classless Routing Behavior: ip classless


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• Route lookup process when ip classless is in useo If classless routing behavior in effect then

Search level 1 routes Supernet routes Checked first

o If a match exists then forward packet Default routes Checked second

o If there is no match or no default route then the Packet is dropped

• Classless Routing Behavior – Search Process• Router begins search process by finding a match between destination IP and parent

routeo After finding the above mentioned match, then there is a search of the child route

• Classless Routing Behavior – Search Process• If no match is found in child routes of previous slide then

o Router continues to search the routing table for a match that may have fewer bitsin the match

• Classful vs. Classless Routing Behavioro It is recommended to use classless routing behavior

Reason: so supernet and default routes can be used whenever needed

Summary

• Content/structure of a routing tableo Routing table entries

Directly connected networks Static route

Dynamic routing protocolso 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 route

- These are subnets of a network address• Routing table lookup process

o Begins with examining level 1 routes for best match with packet’s destination IPIf the best match = an ultimate route then Packet is forwarded

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

examinedo Level 2 route examination

If a match between destination IP and child route found then Packetforwarded -Else

If Router is using classful routing behavior then Packet is dropped -Else If router is using classless routing behavior then


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Router searches Level 1 supernet & default routes for a match If a match is found then Packet if forwarded -Else Packet is dropped

•

Routing behaviorso This refers to how a routing table is searched

• Classful routing behavioro Indicated by the use of the no ip classless commando Router will not look beyond child routes for a lesser match

• Classless routing behavioro Indicated by the use of the ip classless commando Router will look beyond child routes for a lesser match

Chapter 9 – 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.

EIGRP

• Roots of EIGRP: IGRPo Developed in 1985 to overcome RIPv1’s limited hop counto Distance vector routing protocolo Metrics used by IGRP

bandwidth (used by default) Delay (used by default) reliability load

o Discontinued support starting with IOS 12.2(13)T & 12.2(R1s4)S• EIGRP Message Format

o 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

o EIGRP packet header contains Opcode field Autonomous System number

o EIGRP Parameters contains


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Weights Hold time

• TLV: IP internal containso

Metric fieldo Subnet mask fieldo Destination field

• TLV: IP external containso Fields used when external routes are imported into EIGRP routing process

• Protocol Dependent Modules (PDM) o EIGRP uses PDM to route several different protocols i.e. IP, IPX & AppleTalko PDMs are responsible for the specific routing task for each network layer protocol

• Reliable Transport Protocol (RTP)o Purpose of RTP

Used by EIGRP to transmit and receive EIGRP packets

o 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• EIGRP’s 5 Packet Types

o Hello packets Used to discover & form adjacencies with neighbors

o Update packets Used to propagate routing information

o Acknowledgement packets Used to acknowledge receipt of update, query & reply packets

o Query & Reply packets Used by DUAL for searching for networks Query packets

- Can use Unicast Multicast

Reply packet-

Use only Unicast• Purpose of Hello Protocol

o To discover & establish adjacencies with neighbor routers• Characteristics of hello protocol

o Time interval for sending hello packet Most networks it is every 5 seconds Multipoint non broadcast multi-access networks


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- Unicast every 60 secondso Holdtime

This is the maximum time router should wait before declaring a neighbor

down Default holdtime- 3 times hello interval

• EIGRP Bounded Updateso EIGRP only sends update when there is a change in route statuso Partial update

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

o Bounded update When a route changes, only those devices that are impacted will be notified

of the changeo

EIGRP’s use of partial bounded updates minimizes use of bandwidth• Diffusing Update Algorithm (DUAL)

o Purpose EIGRP’s primary method for preventing routing loops

o Advantage of using DUAL Provides for fast convergence time by keeping a list of loop-free backup

routes• Administrative Distance (AD)

o Defined as the trustworthiness of the source route• EIGRP default administrative distances

o Summary routes = 5o

Internal routes = 90o Imported routes = 170

• Authenticationo EIGRP can

Encrypt routing information Authenticate routing information

• Network Topologyo Topology used is the same as previous chapters with the addition of an ISP

router• EIGRP will automatically summarize routes at classful boundaries

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

o This is a collection of networks under the control of a single authority (referenceRFC 1930)

o AS Numbers are assigned by IANAo Entities needing AS numbers

ISP Internet Backbone providers


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Institutions connecting to other institutions using AS numbers• EIGRP autonomous system number actually functions as a process ID• Process ID represents an instance of the routing protocol running on a router•

Exampleo Router(config)#router eigrp autonomous-system

• The router eigrp commando The global command that enables eigrp is router eigrp autonomous-system

All routers in the EIGRP routing domain must use the same process IDnumber (autonomous-system number)

• The Network Commando Functions of the network command

Enables interfaces to transmit & receive EIGRP updates Includes network or subnet in EIGRP updates

o Example

Router(config-router)#network network-address• The network Command with a Wildcard Mask

o This option is used when you want to configure EIGRP to advertise specificsubnets

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

• Verifying EIGRPo EIGRP routers must establish adjacencies with their neighbors before any

updates can be sent or receivedo Command used to view neighbor table and verify that EIGRP has established

adjacencies with neighbors is

show ip eigrp neighbors • The show ip protocols command is also used to verify that EIGRP is enabled• Examining the Routing Table

o The show ip route command is also used to verify EIGRPo EIGRP routes are denoted in a routing table by the letter “D”o By default , EIGRP automatically summarizes routes at major network boundary

• Introducing the Null0 Summary Routeo Null0 is not a physical interfaceo In the routing table summary routes are sourced from Null0

Reason: routes are used for advertisement purposeso 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

• R3’s routing table shows that the 172.16.0.0/16 network is automaticallysummarized by R1 & R3

EIGRP Metric Calculation

• EIGRP Composite Metric & the K Values


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o EIGRP uses the following values in its composite metric Bandwidth, delay, reliability, and load

o The composite metric used by EIGRP formula used has values K1

K5

- K1 & K3 = 1- all other K values = 0

• Use the sh ip protocols command to verify the K values• EIGRP Metrics

o Use the show interfaces command to view metricso EIGRP Metrics

Bandwidth – EIGRP uses a static bandwidth to calculate metric Most serial interfaces use a default bandwidth value of 1.544Mbos (T1)

• EIGRP Metricso 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

• Reliability (not a default EIGRP metric)o A measure of the likelihood that a link will failo Measure dynamically & expressed as a fraction of 255 the higher the fraction the

better the reliability• Load (not a default EIGRP metric)

o A number that reflects how much traffic is using a linko Number is determined dynamically and is expressed as a fraction of 255

The lower the fraction the less the load on the link• Using the Bandwidth Command

o Modifying the interface bandwidth Use the bandwidth command Example

- Router(config-if)#bandwidth kilobits o Verifying bandwidth

Use the show interface commando Note – bandwidth command does not change the link’s physical bandwidth

• The EIGRP metric can be determined by examining the bandwidth delay• EIGRP uses the lowest bandwidth (BW) in its metric calculation

o Calculated BW = reference BW / lowest BW(kbps) • Delay – EIGRP uses the cumulative sum of all outgoing interfaces

o Calculated Delay = the sum of outgoing interface delays• EIGRP Metric = calculated BW + calculated delay

DUAL Concepts

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

o The best least cost route to a destination found in the routing table• Feasible distance


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o The lowest calculated metric along a path to a destination network• Feasible Successors, Feasibility Condition & Reported Distance• Feasible Successor

o This is a loop free backup route to same destination as successor route• Reported distance (RD)

o The metric that a router reports to a neighbor about its own cost to that network• Feasibility Condition (FC)

o Met when a neighbor’s RD is less than the local router’s FD to the samedestination network

• Topology Table: Successor & Feasible Successor• EIGRP Topology table

o Viewed using the show ip eigrp topology command Contents of table include:

- all successor routes

- all feasible successor routes

• EIGRP Topology Table dissected• Topology Table: No Feasible Successor

o A feasible successor may not be present because the feasibility condition maynot be met In other words, the reported distance of the neighbor is greater than or equal

to the current feasible distance• Finite Sate Machine (FSM)

o An abstract machine that defines a set of possible states something can gothrough, what event causes those states and what events result form thosestates

o FSMs are used to describe how a device, computer program, or routing algorithmwill react to a set of input events

• DUAL FSMo Selects a best loop-free path to a destinationo Selects alternate routes by using information in EIGRP tables

• Finite State Machines (FSM)o To examine output from EIGRP’s finite state machine us the debug eigrp fsm

command

More EIGRP Configurations

• The Null0 Summary Routeo By default, EIGRP uses the Null0 interface to discard any packets that match the

parent route but do not match any of the child routeso 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

• Disabling Automatic Summarization


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o The auto-summary command permits EIGRP to automatically summarize atmajor network boundaries

o The no auto-summary command is used to disable automatic summarization This causes all EIGRP neighbors to send updates that will not beautomatically summarized

- This will cause changes to appear in both routing tables topology tables

• Manual Summarizationo Manual summarization can include supernets

Reason: EIGRP is a classless routing protocol & include subnet mask inupdate

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

subnet-mask• Configuring a summary route in EIGRP• EIGRP Default Routes

o “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 domaino EIGRP & the “Quad zero” static default route

Requires the use of the redistribute static command to disseminate defaultroute in EIGRP updates

• Fine-Tuning EIGRPo

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• Configuring Hello Intervals and Hold Times

o Hello intervals and hold times are configurable on a per-interface basiso 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

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

Summary

• Background & Historyo EIGRP is a derivati

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