CCNA 2 Week 7 Distance Vector Routing

Copyright © 2005 University of Bolton Topics Distance Vector Protocol Issues RIP IGRP

Copyright © 2005 University of Bolton Routing Updates Routing tables are learned dynamically from neighbours Updates occur periodically and when topology changes Each router sends its entire routing table to its neighbours

Copyright © 2005 University of Bolton Routing Loops Net1 Direct FA0/0 Net1 3hop via Rtr BNet1 1Hop via Rtr E Net1 2Hop FA0/0 Net1 2Hop via Rtr A Net1 unreachable X Net1 4hop via Rtr C Net1 5hop via Rtr D

Copyright © 2005 University of Bolton Maximum Hop Count In a routing loop, traffic continuously circulates despite target network being down Routers count to infinity while packets loop DV protocols define infinity as specific maximum number to prevent endless looping If max is 15, networks become unreachable once count is exceeded.

Copyright © 2005 University of Bolton Split Horizon Rule Split horizon is used to avoid loop situation. If a routing update about Network 1 arrives from Router A, Router B or Router D cannot send information about Network 1 back to Router A. Split horizon reduces incorrect routing information and routing overhead.

Copyright © 2005 University of Bolton Route Poisoning If a network goes down, router can set a distance vector one greater than maximum to poison route Once poisoned update propagates other routers are unaffected by incorrect updates Router C sends poison reverse back to E so that it can't learn an incorrect route either

Copyright © 2005 University of Bolton Triggered Updates Protocols like RIP send updates periodically every 30 seconds Triggered updates are sent immediately when the routing table changes, with neighbours also forwarding the changes immediately When used with route poisoning, triggered updates help to ensure unreachable networks are correctly propagated

Copyright © 2005 University of Bolton Holddown Timers When a router receives an update marking a route as down the router marks the route as inaccessible and starts a holddown timer While timer running: –An update from the same neighbour saying route is back then route restored, timer cancelled –An update from a different neighbour with a better metric then the router marks the network as accessible and removes the timer. –If an update is received from a different router with a higher metric before the holddown timer expires, the update is ignored

RIP Protocol

Copyright © 2005 University of Bolton RIP Widely implemented standard Uses a maximum hop count (15) to prevent routing loops Implements features such as holddown timers and split horizon to prevent loops RIP v2 supports extra features

Copyright © 2005 University of Bolton Basic Setup Updates sent every 30 seconds unless triggered Received hop count increased by 1 Routers use best path (lowest hop count)

Copyright © 2005 University of Bolton Classless routing Sometimes a router receives packets destined for an unknown subnet of a network that has directly connected subnets – won't use default Rtr(glaboal)# ip classless forwards packets to best supernet route. A supernet route covers a range of subnets in one entry. e.g /16, is a supernet route for /24 The ip classless command is enabled by default since IOS To disable use the no ip classless

Copyright © 2005 University of Bolton Further configuration Router(config-if)# no ip split-horizon Router(config-router)#timers basicupdate invalid holddown flush [sleeptime ] RIP timer defaults 30s, 180s, 180s, 240s [N/A] Holddown should be just greater than max time to propagate updates around network

Copyright © 2005 University of Bolton Further configuration Turn off broadcasts on unwanted interfaces –BHM(config-router)# passive-interface FA0/0 On networks that don't support broadcast –BHM(config-router)# neighbor ip address

Copyright © 2005 University of Bolton Verify RIP show ip protocols –shows how RIP is configured –details of interfaces –networks being advertised show ip route –contents of routing table –direct and learned routes Additional information: –show interfaceinterface –show ip interfaceinterface –show running-config

Copyright © 2005 University of Bolton Troubleshooting debug ip rip –displays routing updates as received Other debug commands –show ip rip database –show ip protocols {summary} –show ip route –debug ip rip {events} –show ip interface brief

Copyright © 2005 University of Bolton Preventing Updates Route filtering regulates information advertised or received into routing table Only apply to Distance Vector protocols Router(config-router)# passive interface s0/0 –Prevents route table being sent out via interface –Will still accept incoming routing tables

Copyright © 2005 University of Bolton Load Balancing RIP will load balance up to 6 equal cost routes Cost refers to the routing metric –Hop count for RIP –Takes no account of link speed Round-robin balancing

Copyright © 2005 University of Bolton Load Balancing in detail Where several protocols in use, router adds route with lowest administrative distance in table –Effectively a priority list for routing protocols Two routes with the same AD – lowest metric used Same AD and metric – load balancing occurs Limits vary by protocol, can set using: Router(config-router)# maximum-paths [number ]

Copyright © 2005 University of Bolton Static routes Administrator can configure routes manually if none learned – often used for gateway route Allow static routes to be overridden by learned routes by setting a high AD Static routes pointing out an interface are redistributed via RIP Static routes not associated with a network command only sent if redistribute static command used

IGRP Interior Gateway Routing Protocol

Copyright © 2005 University of Bolton Routing metric Uses composite metric –Bandwidth – The lowest bandwidth value in the path –Delay – The cumulative interface delay along the path –Reliability – The reliability on the link toward the destination as determined by the exchange of keepalives –Load – The load on a link toward the destination based on bps show ip route displays five weightings K1-K5 By default bandwidth and delay are used

Copyright © 2005 University of Bolton IGRP routes Interior Between subnets of a route attached to an interface If not subnetted, IGRP does not advertise System Other networks within Autonomous System Derived from direct connections and other routers Exterior Between Autonomous Systems Gateway chosen from list of exterior routes in IGRP routing table

Copyright © 2005 University of Bolton Avoiding Loops IGRP supports –Split horizon –Holddown timers –Poison Reverse Updates Increasing metric by factor of 1.1 triggers poison reverse update to place route in holddown Timers as before include Update, Holddown, Invalid and Flush – default update time is 90s

Copyright © 2005 University of Bolton Configure IGRP BHM(config)# router igrp 101 –101 is the AS number and does vary! BHM(config-router)# network Cancel either command using the no prefix

Copyright © 2005 University of Bolton Debug IGRP show ip route –IGRP routes prefixed with 'I' Useful commands –show interfaceinterface –show running-config –show running-config interfaceinterface –show running-config | begin interfaceinterface –show running-config | begin igrp –show ip protocols

Copyright © 2005 University of Bolton Debug commands show ip protocols show ip route debug ip igrp events debug ip igrp transactions ping traceroute

Copyright © 2005 University of Bolton