routing - once again

Hello

Once again i have question about routing:

--network1---Router1----Router2---network2

RIP protocol used. Normally Router2 sends updates to Router1: network2, metrics=1.

Case1: network2 is disconnected, i DO NOT use route poisoning, and i do not use split horizon. How does that update from Router2 to Router1 looks like ? After that how does update from Router1 to Router2 looks like ?

Case2: network2 is disconnected, i DO use route poisoning, and i do not use split horizon. How does that update from Router2 to Router1 looks like ? After that how does update from Router1 to Router2 looks like ?

Thanx Michal

it does matter - because i still do not understeand what is the difference between systems with and without router poisoning, what is the difference between sending data with metric 16 and not sending data - they both can be interpreted in the same way - network is down. So what route poisoning is really for ? For me it only makes convergence faster... If somebody could resolve these two Cases below, maybe i could understeand it finally.

Thanx for the patience and help :)

Ok. My mistakes, under some circumstances routing loop can still happenned. Let's work this out and see what would happenned.

Let's start from the beginning. At t1, the routing table of Router1 & Router2 looks like this: R1: network1 - connected R2: network2 - connected

Now, at t2 update, both routers exchange the whole routing table: R1: network1 - connected network2 - Via R2 - 1 hop R2: network1 - Via R1 - 1 hop network2 - connected

the network has converged at this point.

Now, Normally R2 will send an update to R1 announcing network2 is down and they both converged to: R1: network1 - connected R2: network1 - via R1 - 1 hop.

But what would happenned if, for some reasons, R1 didn't receive this update? R1 still has the old information: R1: network1 - connected network2 - Via R2 - 1 hop and at the next update, send this information back to R2 (no split-horizon). R2 now has: R2: network1 - Via R1 - 1 hop network2 - Via R1 - 2 hops and at the next update, R2 now send this information back to R1 (again because of no split-horizon). R1 now has: R1: network1 - connected network2 - Via R2 - 3 hops and at the next update, R1 now send this information back to R2. R2: network1 - Via R1 - 1 hop network2 - Via R1 - 4 hops ..... As you can see, without split-horizon, a routing loop has occurred. With RIP, this stops when the hop-count for network2 reached 16 (counting to infinity), which is another feature to stop routing loops.

Route poisoning or poisoned reverse, R1 will advertise back to Router2 that it can reach network2 with hop-count of 16 (infinity) and stop the loop faster.

Hope this helps.

Doan