CIS460 – NETWORK ANALYSIS AND DESIGN

CHAPTER 7 -

Selecting Bridging, Switching, and Routing Protocols

Introduction• In this chapter we are going to look at bridging,

switching, and routing protocol attributes of:– Network Traffic characteristics– Bandwidth, memory, and CPU usage– The approximate number of peer routers or switches

supported– The capability to quickly adapt to changes in an

internetwork– The capability to authenticate route updates for security

reasons

Making Decisions as Part of the Top-Down Network Design Process

• Factors involved in making sound decisions:– Goals must be established– Many options should be explored– The consequences of the decisions should be

investigated– Contingency plans should be made

• Use a decision to match options with goals

Making Decisions as Part of the Top-Down Network Design Process (Cont’d)

• Table 7-1 shows a decision table

• Once decision is made look at it to determine:– What could go wrong– Hs it been tried before– How will customer react– Contingency plans if customer disapproves

• Can use during both logical and physical design phase

Selecting Bridging and Switching Methods

• Decision making is simple because of few options– If includes Ethernet bridges and switches most likely use

transparent bridging with spanning-tree protocol– Might also need a protocol for connecting switches that

support virtual LANs– With Token Ring networks options include source-route

bridging (SRB), source-route transparent (SRT) bridging and source-route switching (SRS)

Characterizing Bridging and Switching Methods

– Bridges operate at Layers 1 and 2 of OSI– Determine how to forward a frame based on

information in Layer 2 header– Bridge does not look at Layer 3 information– Bridge segments bandwidth domains so that devices

do not compete with each other for media access control

– Bridge does forward Ethernet collisions or MAC frames in a Token Ring network

Characterizing Bridging and Switching Methods (Cont’d)

– Bridge does not segment broadcast domains. It sends broadcast packets out all ports

– Bridges normally connect like networks but can be a translation or encapsulating bridge

– A switch is like a bridge only faster– Switches take advantage of fast integrated circuits to

offer very low latency– Switches usually have a higher port density and a

lower cost per port

Characterizing Bridging and Switching Methods (Cont’d)

• Bridges do store and forward

• Switches can be store and forward or cut-through

• Cut-through is faster but more prone to letting runts or error packets through

• On a network that is prone to errors do not use cut-through processing

• Adaptive cut-through switching

Transparent Bridging

• Most common Ethernet environments

• A transparent bridge (switch) connects one or more LAN segments so that end systems on different segments can communicate with each other transparently

• Looks at the source address in each frame to learn location of network devices

• It develops a switching table (Table 7-2)

Transparent Bridging (Cont’d)

• Receives a packet look sup address in switch table

• If no address it sends the frame out every port like a broadcast frame

• Send Bridge Protocol Data Unit (BPDU) frames to each other to build and maintain the spanning tree

• Sends BPDU to a multicast address every two seconds

Source-Route Bridging

• Developed for Token Ring networks in the 80s by IBM

• Uses a source-routing-transparent (SRT) standard

• An SRT bridge can act like a transparent bridge or a source-routing bridge depending on whether source-routing information is included in a frame

• Not transparent if pure SRB is used

Source-Route Bridging (Cont’d)

• Uses explorer frames– All-routes explorer - take all possible paths, take just

one route back– Single-route explorer - takes just one path and

response take all paths or just one back– With single-route explorer frames the spanning-tree

algorithm can be used to determine a single path– Scalability is impacted by amount of traffic when

all-routes explorer frames are used

Source-Route Switching

• SRS is based on SRT bridging

• SRS forwards a frame that has no routing information field

• Learns the MAC addresses of devices on the ring

• Also learns source-routing information for devices on the other side of SRB bridges

Source-Route Switching (Cont’d)

• Benefits– Rings can be segmented without adding new ring

numbers– can be incrementally upgraded to transparent bridging

with minimal disruption or reconfiguration– does not need to learn the MAC addresses of devices on

the other side of source-route bridges– can support parallel source routing paths– can support duplicate MAC addresses

Mixed-Media Bridging

– Mixture of Token Ring, FDDI and Ethernet bridging– Encapsulating bridging is simpler than translation

bridging but is only appropriate for some network topologies

– Encapsulating bridge encapsulates an Ethernet frame inside an FDDI or Token ring frame for transversal across a backbone network that has no end systems

Mixed-Media Bridging (Cont’d)

• Support for end systems on a backbone then need to use translation bridging which translates from one data-link-layer protocol to another– Problems

• Incompatible bit ordering

• Embedded MAC addresses

• Incompatible maximum transfer unit (MTU) sizes

• Handling of exclusive Token Ring and FDDI functions

• No real standardization

Mixed-Media Bridging (Cont’d)

• While FDDI is a common choice for backbone networks in campus network designs to avoid translating Ethernet and FDDI frames should use 100-Mbps Ethernet or Gigabit Ethernet on backbone segments

Switching Protocols for Transporting VLAN Information

• When VLANs are implemented in a switched network the switches need a method to make sure intra-VLAN traffic goes to the correct segments

• Accomplished by tagging frames with VLAN information

• two tagging methods:– adaptation of the IEEE 802.10 security protocol– Inter-Switch Link (ISL) protocol

IEEE 802.10

• A security specification used as a way of placing VLAN identification (VLAN ID) in a frame

• Inserted between the MAC and LLC headers of the frame

• The VLAN ID allows switches and routers to selectively forward packets to ports with the same VLAN ID

• VLAN ID removed from frame when forwarded to destination segment

Inter-Switch Protocol

• Another method for maintaining VLAN information as traffic goes between switches

• Developed to carry VLAN information on a 100-Mbps Ethernet switch-to-switch or switch-to-router link. Can carry multiple VLANs

• ISL link is call a trunk. A trunk is a physical link that carries the traffic of multiple VLANs between two switches or between a switch and a router. Allows VLANs to extend across switches

VLAN Trunk Protocol

• Some networks have a combination of different media types

• VLAN trunk protocol (VTP) allows a VLAN to span the different technologies by automatically configuring a VLAN across a campus network regardless of media type

• VTP is a switch-to-switch and switch-to-router VLAN management protocol that exchanges VLAN configuration changes as they are made to the network

Selecting Routing Protocols

• A routing protocol lets a router dynamically learn how to reach other networks and exchange this information with other routers or hosts

• Selecting routing protocols is harder than selecting bridging protocols because there are so many

• Made easier using a table such as 7-1 to pick the best one

Characterizing Routing Protocols

• General goal to share network reachability information among routers

• Some send complete other only an update• Differ in scalability and performance

characteristics– Many are designed for small networks– Static environment– Some are meant for connecting interior campus

networks

Distance-Vector Versus Link-State Routing Protocols

• Two major classes: distance-vector and link-state

• Distance-vector protocols

– IP Routing Information Protocol (RIP) Version 1 and 2

– IP Interior Gateway Routing Protocol (IGRP)

– Novell NetWare Internetwork Packet Exchange Routing Information Protocol (IPX RIP)

– AppleTalk Routing Table Maintenance Protocol (RTMP)

– AppleTalk Update-Based Routing Protocol (AURP)

– IP Enhanced IGRP

– IP Border Gateway Protocol (BGP) (path-vector)

Distance-Vector Versus Link-State Routing Protocols (Cont’d)

• Vector means distance or course. A distance-vector includes information on the length of the course. Many use hop count

• A hop count specifies the number of routers that must be traversed

• Maintains a distance-vector routing table that lists know networks and the distance to each.

• Sends table to all neighbors, or an update after first transmission

Distance-Vector (Cont’d)– Split Horizon, Hold-Down, and Poison-Reverse

Features• Split-horizon technique - sends only routes that are

reachable via other ports

• Hold-down timer - new information about a route to a suspect network is not believed right away. A standard way to avoid loops

• Poison-reverse messages - way of speeding convergence and avoiding loops. When a router notices a problem it can immediately send a route update that specifies the destination is no longer reachable

Link-State Routing Protocols

– Do not exchange routing tables– Exchange information about the status of their

directly connected links using periodic multicast messages

– Each router builds its own routing table– Protocols

• IP Open Shortest Path First (OSFP)

• IP Intermediate System-to-Intermediate System (IS-IS)

• NetWare Link Services Protocol (NLSP)

Link-State Routing Protocols (Cont’d)

• Converge more quickly

• Less prone to routing loops

• Require more CPU power and memory

• More expensive to implement and support

• Harder to troubleshoot

Routing Protocol Metrics

• Used to determine which path is preferable when more than one path is available

• Vary on which metrics are supported

• Distance-vector use hop count

• Newer protocols take into account delay, bandwidth, reliability and other factors

• Metrics can effect scalability

Hierarchical Versus Non-Hierarchical Routing Protocols– Some routing protocols do not support hierarchy– Normally all routers perform same tasks– Hierarchical protocols assign different tasks to

different routers and group routers in areas– Some routers communicate with local routers in

the same area and other routers have the hob of connecting areas, domains, or autonomous systems

Interior Versus Exterior Routing Protocols

• Interior protocols, such as RIP, OSPF, and IGRP are used by routers within the same enterprise or autonomous

• Exterior such as BGP perform routing between multiple autonomous systems.

Classful Versus Classless Routing Protocols

– A classful routing protocol always considers the IP network class

– Address summarization is automatic by major network number and discontiguous subnets are not visible to each other

– Classless protocols transmit prefix-length or subnet mask information with IP network addresses. The IP address can be mapped so that discontinuous subnets and VLSM are supported

Dynamic Versus Static and Default Routing

• Static routes are often used to connect to a stub network

• A stub network is a part of an internetwork that can only be reached by one path

• Internal routers can simply be configured with a default route that points to the ISP

Scalability Constraints for Routing Protocols

• Consider customer’s goals for scaling the network to a larger size

• There are a number of questions that relate to scalability that should be answered

• They can be answered by watching routing protocol behavior with a protocol analyzer and by studying the relevant specifications

Routing Protocols Convergence

• Convergence is the time it takes for routers to arrive at a consistent understanding of the internetwork topology after a change takes place

• Understand the frequency of changes, links that fail often, etc

• Convergence time is a critical design constraint

Routing Protocols Convergence (Cont’d)

– Convergence starts when a router notices a link has failed

– If a serial link fails it can start immediately. If it uses keepalive frames it starts convergence after it has been unable to send two or three keepalive frames

– If use hello packets and the hello timer is shorter than the keep alive timer then routing protocol it can start convergence sooner

IP Routing

• Most common protocols are RIP, IGRP, Enhanced IGRP, OSPF, and BGP

Routing Information Protocol

– The first standard routing protocol developed for TCP/IP environments

– It is a distance-vector protocol that features simplicity and ease-of-troubleshooting

– Uses a hop count to measure the distance to a destination. Cannot be more than 15 hops

– RIPv2 developed to address some of the scalability and performance problems with Version 1

Interior Gateway Routing Protocol

• Meet needs of customers requiring a robust and scalable interior routing protocol

• Uses composite metric based on: bandwidth, delay, reliability, and load

• Load balances over equal-metric paths and non-equal-metric paths. (3 to 1)

• Has a better algorithm for advertising and selecting a default rout than RIP

Enhanced Interior Gateway Routing Protocol

– Meet the needs of enterprise customers with large, complex, multiprotocol internetworks

– Goal is to offer quick convergence on large networks. Diffusing update algorithm (DUAL) guarantees a loop-free topology

– The router develops a topology table that contains all destinations advertised by neighboring routers. It can scale to thousands of nodes

Open Shortest Path First

– Open standard supported by many vendors– converges quickly– authenticates protocol exchanges to meet

security goals– supports discontiguous subnets and VLSM– sends multicast frames vice broadcast frames– does not use a log of bandwidth– can be designed in hierarchical areas

Open Shortest Path First (Cont’d)

– Propagates only changes– accumulate link-state information to calculate the

shortest path to a destination– all routers run the same algorithm in parallel– Allows sets of networks to be grouped into areas– A contiguous backbone area, called Area ) is

required– Assign network numbers in blocks that can be

summarized

Border Gate Protocol

• iBGP used at large companies to route between domains

• EBGP is often used to multihome an enterprise’s connection to the Internet

• Main goal is to allow routers to exchange information on paths to destination networks

Apple Talk Routing

• Three options:

• Routing Table Maintenance Protocol (RTMP)

• AppleTalk Update-Based Routing Protocol (AURP)

• Enhanced IGRP for AppleTalk

• RTMP is most common because it is easiest to configure and is supported by most vendors

Routing Table Maintenance Protocol

• Routing table sent every 10 seconds using split horizon

• Works closely with Zone Information Protocol (ZIP)

• Checks routing table updates and sends ZIP query

Using Multiple Routing and Bridging Protocols

• Important to realize you do not have to use the same routing and bridging protocols throughout the internetwork

• To merge old networks with new networks it is often necessary to run more than one routing or bridging protocol

• Solutions include source-route transparent bridging, external routes in OSPF and RIP2

Redistribution between Routing Protocols

– Redistribution allows a router to run more than one routing protocol and share routes among routing protocols

– Network administrator must configure redistribution by specifying which protocols should insert routing information into other protocol’s routing tables

– A router can learn about a destination from more than one protocol

Integrated Routing and Bridging

• CISCO offers support for IRB which connects VLANs and bridged networks to routed networks within the same router

• One advantage of IRD is that a bridged IP subnet or VLAN can span a router

Summary

• Deciding on the right bridging, switching, and routing protocols for your customer will help you select the best switch and router products for the customer