Oh. I think I missed the point the first time around. You are stating as known fact that Cisco RIP definitely does not reverse poison the route immediately upon learning the route (what I would think of as a classic definition of poison reverse). This must be one of those rare ambiguous uses of terms I have heard occasionally happens in tech industry.
Thanks again. Priscilla Oppenheimer wrote: > > These terms aren't defined authoritatively anywhere, but > thinking about the > English-language meaning of the terms does help. Poisoning a > route simply > means stating that a route is unreachable. If it's sent in the > reverse > direction, then it's poison reverse. > > Poison reverse is usually used as a form of split horizon in a > proactive > manner, as I described for EIGRP. But the term doesn't have to > be used that > way. > > Cisco claims to support poison reverse for RIP. But the > behavior is as I > described and not proactive (at least on routers; I haven't > studied routing > switches). > > For any definition that you find that describes precise > behavior, I can > find another one that describes it differently! ;-) The terms > are used in a > variety of ways. > > Priscilla > > At 01:25 PM 1/28/02, s vermill wrote: > >Priscilla, > > > >Isn't there a difference between poison reverse (which is a > variation on > >split horizon) and route poisoning? I thought poison reverse > took place > >each time a route was learned. For example, router A > advertises network 1 > >to router B. Router B immediately poison reverses the route > to router A. > >Route poisoning only happens in the triggered fashion that you > described in > >your post. Or so I thought. > > > >Thanks, > > > >Scott > > > >Priscilla Oppenheimer wrote: > > > > > > 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 > ________________________ > > Priscilla Oppenheimer > http://www.priscilla.com > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=33487&t=33402 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
