The "poison reverse" term is sometimes used to mean sending a poisoned
route outwards. In that case it doesn't break the rules of split horizon
and it's just a way of explicitly saying this route is no longer reachable,
rather than simply omitting the route from the list. Sometimes it is sent
as a "triggered" update as soon as the problem is confirmed, rather than
waiting for the next regularly scheduled updated.
More often, the "poison reverse" term means going back the other way and
it's not related to a problem. It's a proactive method of avoiding loops,
as you said. Router B reports to Router A that a network that it reaches
via Router A has a distance of infinity. This avoids Router A ever sending
Router B a packet for that network.
Split horizon has the same effect. Router B would never tell Router A that
it could get to that network anyway. There are some cases with certain
topologies and an unfortunate timing of events where split horizon fails to
do the job. Draw three routers in a triangle and think about what
happens... Having been told explicitly not to believe me saying I can get
there when it's you that told me this, may avoid the problem.
Also, keep in mind that distance-vector routing protocols had various
problems in the early days that could be catastrophic. To avoid problems,
many overlapping solutions were developed. They may seem like overkill but
avoiding problems was so important, nobody worried too much about overkill.
Priscilla
At 04:24 PM 5/25/01, Cisco Boy wrote:
>So for poison reverse, does it mean that it's
>temporarily breaking the rules of "split horizon" bye
>sending the route advertisement back (on the interface
>that it received it from) to the original router who
>sent it?
>
>If that's the case. What would cause this process to
>occur? I'm looking for a detailed answer and from
>reading several books, I'm getting different answers.
>
>
>I understand how Split Horizon works, but Poison
>Reverse puzzles me. I'm trying to figure out what
>process leads to it and the exact details of what's
>going on when it does happen. I completely understand
>the end result though, of how it takes a proactive
>stance to prevent routing loops by sending an
>unreachable update.
>
>
>
>--- Priscilla Oppenheimer wrote:
> > At 01:02 PM 5/25/01, you wrote:
> > >Newbie question
> > >
> > >Is the max hop count for rip 15 and the poison
> > reverse infinity count 16?
> > >Thanks
> >
> > The max hop count is 15, as you know. If a router
> > advertises a route as
> > being 16 hops, it's saying that the distance to the
> > network is "infinity,"
> > in other words it's unreachable. These routes are
> > sometimes considered
> > called "poisoned routes." If a router sent such a
> > route advertisement in
> > the reverse direction, back to the router that
> > originally advertised it,
> > this could be considered poison reverse.
> >
> > Poison reverse also happens during normal operation
> > when there aren't any
> > problems. Check out EIGRP behavior. With EIGRP, when
> > Router A sends a route
> > to Router B, Router B sends back a response saying
> > that the route is not
> > reachable. Router B isn't saying there is a problem.
> > It's telling Router A,
> > don't get there through me, you have better
> > knowledge of this network than
> > I do. This avoids loops.
> >
> > Priscilla
> > >FAQ, list archives, and subscription info:
> > >http://www.groupstudy.com/list/cisco.html
> > >Report misconduct and Nondisclosure violations to
> > [EMAIL PROTECTED]
> >
> >
> > ________________________
> >
> > Priscilla Oppenheimer
> > http://www.priscilla.com
> > FAQ, list archives, and subscription info:
> > http://www.groupstudy.com/list/cisco.html
> > Report misconduct and Nondisclosure violations to
>[EMAIL PROTECTED]
>
>
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