Cisco does actually support a form of poison reverse with RIPv1 and v2. It's not proactive, but it still fits the definition.
When a router loses a route to a network behind it, it announces that it can't get to the network by sending a RIP update that lists the network with a distance of 16. The router does this quickly, without waiting for the next update timer. It sends a triggered update. I have observed that other routers then also say, "hey don't use me to get there either." These are routers that never could have gotten there anyway without the help of the other router. Their only path was in the reverse direction from the networks that they advertise. An example might help: ---network1----Router A-----network 2------Router B-----network 3--- Router A loses its route to network 1. Router A sends a triggered update listing network 1 as unreachable (hop count = 16). Router B then also sends an update listing network 1 as unreachable (hop count = 16). That could be considered a poison reverse feature. Note that this isn't as proactive as some forms of poison reverse. If this were EIGRP, for example, as soon as Router A announced it could get to network 1, Router B would send an update saying its distance to network 1 is infinity (delay = max). It proactively tells Router A not to ever use Router B to get to network 1. Router B says it is not a feasible successor for that network. That's definitely poison reverse. Cool, eh? Bottom line: Cisco's implementation of RIP (and of course, EIGRP) has always been a bit more advanced than the textbook descriptions of a distance-vector algorithm. Priscilla At 11:07 PM 1/27/02, Pierre-Alex GUANEL wrote: >Cisco does not seem to support poison reverse for RIP and RIP version 2. > >Do you know network vendors who do? > >Pierre-Alex ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=33467&t=33402 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
