Chapter 08

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CHAPTERCHAPTEREIGHTEIGHT

Routing ProtocolsRouting Protocols

ObjectivesObjectives

• Differentiate between nonroutable, routed and routing protocols

• Define Interior Gateway Protocols, Exterior Gateway Protocols, distance-vector routing protocols, and link-state routing protocols

• Explain the concepts of count-to-infinity, split horizon, split horizon with poison reverse, and hold-down timers

• Describe, configure, and monitor interior routing protocols RIP and IGRP

• Explain static routing and administrative distance• Configure static routing and default routes

Nonroutable ProtocolsNonroutable Protocols

Figure 8-1: Early network model using coaxial cable

Nonroutable ProtocolsNonroutable Protocols

• Peer-to-peer networks– Small networks in which each computer can give

and receive network services

• Nonroutable protocols– Protocols that do not contain Network layer

addressing and therefore can pass between multiple networks

Routed ProtocolsRouted Protocols

• Have packet headers that can contain Network layer addresses• Figure 8-2 shows a typical internetwork within which routed protocols

are used

Figure 8-2: Common internetwork


• Transmission Control Protocol/Internet Protocol (TCP/IP)– Routed protocol stack developed in late 1960s for use on

precursor to the Internet

• Internetwork Packet Exchange/Sequence Packet Exchange (IPX/SPX)– Routed protocol stack developed by Novell for use with the

Netware network operating system

• Logical addresses– Layer 3 addresses that allow routed protocols to determine

which network a particular host is on


Figure 8-3: Common internetwork with IP addresses


• Protocols used by routers to make path determination choices and to share those choices with other routers

Table 8-1: Conceptual route table


• Hop count– Number of routers a packet must pass through to

reach a particular network

• A metric is a value used to define the suitability of a particular route

• An autonomous system (AS) is a group of routers under the control of a single administration


Figure 8-4: Big Tin Inc.’s AS


• Routing protocols come in two major categories:– Interior Gateway Protocols (IGPs)

• Routing Information Protocol (RIP), Interior Gateway Routing Protocol (IGRP), Enhanced Interior Gateway Routing Protocol (EIGRP), and Open Shortest Path First (OSPF) are examples of IGPs

– Exterior Gateway Protocols (EGPs)• Border Gateway Protocol (BGP) and Exterior Gateway

Protocol are examples of EGPs

Types of IGP Routing Protocols:Types of IGP Routing Protocols:Distance-Vector Routing ProtocolsDistance-Vector Routing Protocols

• Broadcast their entire routing table to each neighbor router at predetermined intervals

Figure 8-5: Distance-vector routing protocol process

Types of IGP Routing Protocols:Types of IGP Routing Protocols:Distance-Vector Routing ProtocolsDistance-Vector Routing Protocols• Routing by rumor

– The learning of routes through second-hand information, and not directly from the router experiencing the change

– One of the main problems with distance-vector routing protocols

• Convergence– A state where all routers on the internetwork share

a common view of the internetwork routes


Figure 8-6: Distance-vector convergence example

Types of IGP Routing Protocols:Types of IGP Routing Protocols:Distance-Vector Routing ProtocolsDistance-Vector Routing Protocols• Problems, such as routing loops, can occur

with distance-vector protocols if control measures are not put in place

• Routing loops – Often referred to as count-to-infinity problems

– Network state in which packets are continually forwarded from one router to another in an attempt to find the destination network


Figure 8-7: Distance-vector convergence problems

Types of IGP Routing Protocols:Types of IGP Routing Protocols:Distance-Vector Routing ProtocolsDistance-Vector Routing Protocols• Defining a maximum

– One of the easiest ways to limit count-to-infinity problems

– Technique used with distance-vector routing protocols to prevent packets from bouncing infinitely throughout an internetwork

• If you assign a packet a maximum hop count, it cannot bounce infinitely around the internetwork

Types of IGP Routing Protocols:Types of IGP Routing Protocols:Distance-Vector Routing ProtocolsDistance-Vector Routing Protocols• Two other common ways to prevent routing

loops with distance-vector routing protocols:– Split horizon

• A router will not send an update for a route via an interface from which it originally received knowledge of that route

– Split horizon with poison reverse• A split horizon in which the router responds to attempts

to updated a route with an update that marks the route in contention as unreachable

Types of IGP Routing Protocols:Types of IGP Routing Protocols:Distance-Vector Routing ProtocolsDistance-Vector Routing Protocols• Hold-down timer

– Common technique used to stop routing loops– Allows a router to place a route in a state where it

will not accept any changes to that route– Configurable by the network administrator– Can prevent improper route information from

being propagated throughout the network

Types of IGP Routing Protocols:Types of IGP Routing Protocols:Link-State Routing ProtocolsLink-State Routing Protocols

• Routers configured with a link-state routing protocol use link-state advertisements (LSAs) to inform neighbor routers on the internetwork– Instead of sending entire routing tables, LSAs contain

only local links for the advertised router

– The Shortest Path First (SPF) algorithm uses the link information to compute the routes

– CPU resources are used instead of bandwidth


• Link-state packets (LSPs)– Allow every router in the internetwork to share a

common view of the internetwork’s topology

• Flood– Process of broadcasting packets onto a network

• Triggered updates– Occur due to network topology changes


Figure 8-8: Link-state advertisements


Table 8-2: Major characteristics of distance-vector and link-state routing protocols

Routing Information ProtocolRouting Information Protocol

• A distance-vector routing protocol• Has a maximum hop count of 15• Hop count is the only metric available for path

selection• Broadcasts entire routing table to neighbors

every 30 seconds• Capable of load balancing• Easy to configure

Routing Information ProtocolRouting Information Protocol

• To install RIP on a Cisco router using TCP/IP, you must perform the following two tasks:– Enable RIP

– Configure RIP for each major network you wish to advertise

• You need only to configure major network numbers with RIP– RIP does not maintain subnet mask information

within routing tables it produces

Enable RIP RoutingEnable RIP Routing

Figure 8-9: Sample IP network

Configuring RIP Routing for Each Configuring RIP Routing for Each Major NetworkMajor Network

Figure 8-10: Global configuration mode


• Administrative distance– Value used to determine reliability of a particular route

Figure 8-11: Configuring RIP


Figure 8-12: Output from the show ip route command


Table 8-3: Administrative distances

Show IP Protocol and debug IP Show IP Protocol and debug IP Rip CommandsRip Commands

Figure 8-13: Output from the show ip protocol command

Show Ip Protocol and Debug Ip Show Ip Protocol and Debug Ip Rip CommandsRip Commands

• Flush interval– The time at which a route will be totally removed

from the routing table if no updates are received

• The debug ip rip command—like all debug commands—should only be used when troubleshooting RIP


Figure 8-14: Output from the debug ip rip command


Figure 8-15: Output from the show ip route command


Figure 8-16: New output from the show ip route command


Figure 8-17: RIP problems caused by hop count reliance

Interior Gateway Routing ProtocolInterior Gateway Routing Protocol(IGRP)(IGRP)

• Proprietary distance-vector routing protocol created by Cisco

• Solves some of the problems associated with RIP

• A larger hop count allows IGRP to be used in larger networks

• IGRP does not use hops as a metric by default


• Metrics that can be configured for IGRP:– Hops– Load– Bandwidth– Reliability– Delay– MTU


Figure 8-18: Commands used to configure IGRP


Figure 8-19: The show ip route command with IGRP


Figure 8-20: The show ip protocol command with IGRP


Figure 8-21: Output from the debug ip igrp command

Static RoutingStatic Routing

• Stub router– Router with only one route

• Stub network– Network with one route to the Internet

• Static route– Route manually added by a network administrator

to the routing table of a router

Adding Static RoutesAdding Static Routes

Figure 8-22: Routing table for RouterC

Changing Administrative DistanceChanging Administrative Distance

• IP route command also allows you to configure an administrative distance

• Static routes have a much lower default metric than routes learned by dynamic routing protocols– A static route is considered to be a preferred rout since someone

took the trouble to enter it

• If you want the static route to be a back up route to one learned via a dynamic routing protocol, set the administrative distance of the static route higher than that of the default dynamic routing protocol

Configuring a Default RouteConfiguring a Default Route

• Default route– Static route that directs all traffic not specified

anywhere else in the routing table to a particular route

– Configured by the administrator

– Sometimes called a quad zero route

Configuring a Default RouteConfiguring a Default Route

Figure 8-23: Default route example

Chapter SummaryChapter Summary

• Protocols vary in their functions• Nonroutable protocols are designed to be used in

small networks without the need for Network layer addressing

• Routed protocols were designed with the ability to move between multiple networks via Network layer addressing

• Routing protocols are classed in two major groups:– Interior Gateway Protocols– Exterior Gateway Protocols


• Interior routing protocols are divided into distance-vector routing protocols and link-state routing protocols

• Distance-vector protocols periodically broadcast entire routing tables to neighbor routers

• Link-state protocols broadcast link updates to neighbor routers on the internetwork upon startup and when network topology changes

• Two common distance-vector IGPs are RIP and IGRP


• RIP is an easy-to-configure routing protocol that uses hop count as its sole metric

• IGRP is not limited to using hop counts as its sole metric

• Static routes are used to conserve bandwidth and lower memory and CPU load on a router while still allowing for correct routing table creation

• Static routes give administrators control and flexibility in path selection in a network

Chapter 08

Technology

routing protocols protocols

types of igp routing

distancevector protocols

interior routing protocols

static routing

routed protocols figure

nonroutable protocols

nonroutable protocols