That's correct, that it advertises a route out the same interface that it
learned it on. The only difference is that it advertises the route at a
cost of 15. When the receiving router gets the update, it adds 1 to the hop
count, putting it up to 16 (unreachable).
Why it does this is the same reason why split horizon does what it does:
loop avoidance.
----- Original Message -----
From: Cisco Boy
To: [EMAIL PROTECTED]
Sent: Friday, May 25, 2001 4:24 PM
Subject: Re: Split Horizon & Poison Reverse [7:5887]
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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