Refer to the exhibit.
The network shown in the exhibit is running the RIPv2 routing protocol. The
network has converged, and the routers in this network are functioning properly. The
FastEthernet0/0 interface on R1 goes down. In which two ways will the routers in this network
respond to this change? (Choose two.)
A.
All routers will reference their topology database to determine if any backup routes to the
192.168.1.0 network are known.
B.
Routers R2 and R3 mark the route as inaccessible and will not accept any further routing
updates from R1 until their hold-down timers expire.
C.
Because of the split-horizon rule, router R2 will be prevented from sending erroneous
information to R1 about connectivity to the 192.168.1.0 network.
D.
When router R2 learns from R1 that the link to the 192.168.1.0 network has been lost, R2 will
respond by sending a route back to R1 with an infinite metric to the 192.168.1.0 network.
E.
R1 will send LSAs to R2 and R3 informing them of this change, and then all routers will send
periodic updates at an increased rate until the network again converges.
Explanation:
When Fa0/0 on R1 goes down, R1 will try to inform with R2 that its Fa0/0 interface is currently down. R2 in turn will inform to R3 that Fa0/0 of R1 is down. The split-horizon rule states that “a router never sends information about a route back in same direction which is original information came”. It means when R1 sends information about its downed network 192.168.1.0, R2 is not allowed to send back that information to R1 -> C is correct.
But maybe you will ask “Why answer D is also correct when it seems contradictory to answer C?” Yes, it is really contradictory! This is called the “Poison Reverse” rule:
The poison reverse rule overwrites split horizon rule. For example, if router R2 receives a route poisoning of network 192.168.1.0 from router R1 then router R2 will send an update back to router R1 (which breaks the split horizon rule) with the same poisoned hop count of 16. This ensures all the routers in the domain receive the poisoned route update.
Notice that the “Poison Reverse” doesn’t send erroneous information to R1 but just only one message to make sure R1 is working correctly.
For your information, answer B is not correct because if R2 and R3 get an update with a better metric than the originally recorded metric (1 for R2 and 2 for R3) within the hold-down timer period, the hold-down timer is removed and data can be sent to that network. It means that now R2 and R3 have a better way to reach R1.