Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 186 RIP Loop Avoidance Mechanisms RIP, as a Distance Vector routing protocol, is susceptible to loops. Let’s assume no loop avoidance mechanisms are configured on either router. If the 172.18.0.0 network fails, Router B will send out an update to Router A within 30 seconds (whenever its update timer expires) stating that route is unreachable (metric = 16). But what if an update from Router A reaches Router B before this can happen? Router A believes it can reach the 172.18.0.0 network in one hop (through Router B). This will cause Router B to believe it can reach the failed 172.18.0.0 network in two hops, through Router A. Both routers will continue to increment the metric for the network until they reach a hop count of 16, which is unreachable. This behavior is known as counting to infinity. How can we prevent this from happening? There are several loop avoidance mechanisms: Split-Horizon – Prevents a routing update from being sent out the interface it was received on. In our above example, this would prevent Router A from sending an update for the 172.18.0.0 network back to Router B, as it originally learned the route from Router B. Split-horizon is enabled by default on Cisco Routers. Route-Poisoning – Works in conjunction with split-horizon, by triggering an automatic update for the failed network, without waiting for the update timer to expire. This update is sent out all interfaces with an infinity metric for that network. Hold-Down Timers – Prevents RIP from accepting any new updates for routes in a hold-down state, until the hold-down timer expires. If Router A sends an update to Router B with a higher metric than what is currently in Router B’s routing table, that route will be placed in a hold-down state. (Router A’s metric for the 172.18.0.0 network is 1; while Router B’s metric is 0). * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
CCNA Study Guide v2.62 – Aaron Balchunas 187 RIP Passive Interfaces It is possible to control which router interfaces will participate in the RIP process. Consider the following scenario. Router C does not want to participate in the RIP domain. However, it still wants to listen to updates being sent from Router B, just not send any updates back to Router B: RouterC(config)# router rip RouterC(config-router)# network 10.4.0.0 RouterC(config-router)# network 10.2.0.0 RouterC(config-router)# passive-interface s0 The passive-interface command will prevent updates from being sent out of the Serial0 interface, but Router C will still receive updates on this interface. We can configure all interfaces to be passive using the passive-interface default command, and then individually use the no passive-interface command on the interfaces we do want updates to be sent out: RouterC(config)# router rip RouterC(config-router)# network 10.4.0.0 RouterC(config-router)# network 10.2.0.0 RouterC(config-router)# passive-interface default RouterC(config-router)# no passive-interface e0 * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
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Cisco CCNA Study Guide - Router Alley

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