RIP V2 route poision

masterk

Hello all

In the official Cisco book, it states that if a RIP router sends a flash update to poision a route with a metric of 16, receiving routers will place that route in hold down, then will flush etc......

Thats great but..... in testing that doesnt happen. I shut a port on router B, it sends a flash update to Router A, and router A flushes the route from the routing table instantly.

So it seems the theory isnt matching up with the practical?

Any ideas?

Thanks in advance.

EdTheLad

When you shut the interface on RB, RB will send an update to RA with a metric 16, instantaneously RA will flush the rib and put this prefix in a holddown state, while in holddown if another update is received for this prefix with a better metric i.e. <16 the prefix will be ignored until the holddown timer expires.Once the holddown timer expires and an update with this prefix arrives, it will enter the rib.

masterk

Hi thanks for the reply

Yes thats what i expect to happen, but in practice it isnt, the route vanishes from the routing table on router A, no hold-down, just gone. Any ideas?

EdTheLad

Reread my post i don't think you understand,the behavior you are seeing is correct.

This is the test you should run.Configure rip to run between RA,RB and RC, disable auto-summary, enable version 2.
On RA config an interface with 1.1.1.1/32, advertise this under rip i.e. network 1.0.0.0
Configure an interface with 1.1.1.1/32 on RC, don't apply this network to RIP.
Verify that the 1.1.1.1/32 is present in the rib on RB and learned via RA.Shutdown the interface on RA, the route will be removed from the rib on RB.Now quickly add the statement "network 1.0.0.0" under the rip process on RC.RB will receive this update from RC but wont populate the rib until the holddown timer for this prefix expires.Run some debug commands on RC to see whats happening.

EdTheLad

Just thinking,maybe a metric of 16 wont trigger the holddown timer, i haven't labbed this up so probably receiving a metric of 16 will just flush the route.If by shutting the link down you cant reproduce what i said earlier, try using an offset-list to increase the metric but just make sure the metric is < 16.So if RB has a route to the prefix via RA metric 2, add an offset-list to the egress interface of RA,add offset 5.RB will receive an update to that same prefix from the same next-hop with an increased metric and hence initiate the holddown timer.

EdTheLad

Disregard what i said earlier, i think Cisco have changed the behavior of RIP in the newer ios.As far as i've seen a route will only go into holddown if the invalid timer expires.

masterk

Ed thanks for all the replies, your latest reply is very interesting. I was pulling my hair out trying to figure out what i was doing wrong!! Did you manage to replicate what i was talking about?

How did you get to the conclusion about the changes in later IOS versions?

Thanks!

masterk

check this out:

https://learningnetwork.cisco.com/thread/20422

"If R1 simply stops talking, and R2 hits the invalid timer for that route, it will begin the holdown, and send a poisoned route to R3. Because R3 got the poisoned route from its downstream router (R2), R3 will delete the route, and not go into hold down."

The books are wrong!!!