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Pages (2): [1] 2 »
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Looking for more details on Poison Reverse and others
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Bernie
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Registered: Aug 2000
Location: Raleigh, NC
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Route Poisoning, Triggered Updates, Poison Reverse....
I need more clarification on what these do, how they do it. I have read them in the Sybex book and I'm having a tough time locating a good page with detailed info on the Cisco site. Can anybody send me a link?
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 03-01-01 04:25 PM
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Azam
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03-01-01 04:28 PM
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Bernie
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03-01-01 07:01 PM
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Azam
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03-01-01 07:02 PM
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doctorcisco
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quote:
Originally posted by Bernie
Route Poisoning, Triggered Updates, Poison Reverse....
I need more clarification on what these do, how they do it. I have read them in the Sybex book and I'm having a tough time locating a good page with detailed info on the Cisco site. Can anybody send me a link?
Route Poisoning -- when a route drops, the router(s) send out an update for that route with an infinite metric, marking it unreachable. "Infinite" in RIP is 16 hops, for instance.
Triggered Updates -- instead of waiting for the regularly scheduled update, the routers send out a "flash update" of a topology change as soon as they know about it.
Poison reverse -- a route poisoning that is sent out ALL of a router's interfaces, even when it violates the split horizon rule.
All 3 are used together in IGRP and RIP to avoid routing loops and speed convergence.
HTH,
doctorcisco
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03-02-01 12:22 AM
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dmaftei
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I hate to disagree with the Doctor, but afaik "poison reverse" is short for "split horizon with poison reverse".
Plain split horizon says "do NOT send back an update out the interface on which it was received";
(split horizon with) poison reverse says "DO send back the update, but with infinite metric".
Cheers!
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03-02-01 02:34 AM
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doctorcisco
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quote:
Originally posted by dmaftei
I hate to disagree with the Doctor, but afaik "poison reverse" is short for "split horizon with poison reverse".
Plain split horizon says "do NOT send back an update out the interface on which it was received";
(split horizon with) poison reverse says "DO send back the update, but with infinite metric".
Cheers!
I'll gladly accede to dmaftei's explanation of the term. In either case, unless I'm missing something, the point is much the same ... poison reverse sends route poisoning updates back toward their source, even though the routing protocol is using split horizon. Bad news goes everywhere; good news is subject to split horizon rules. 
doctorcisco
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03-02-01 03:50 AM
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dmaftei
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quote:
Originally posted by doctorcisco
In either case, unless I'm missing something, the point is much the same ... poison reverse sends route poisoning updates back toward their source, even though the routing protocol is using split horizon.
Not really. Poison reverse is a reaction to a received update: you get a route to X, you do some crunching and sooner or later you'll send your update containing X on the other interfaces, and you poison X on the interface on which it was received. You react because of a change that happened in another place.
On the other hand with route poisoning you're the one that causes the change. Say your administrator removes a static route (that used to be advertised). You then set the metric for that route to "infinity" (you poison the route), and send an update on all your interfaces.
Am I confusing people, rather than clarifying?!
Cheers!
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03-02-01 10:54 PM
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doctorcisco
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quote:
Originally posted by dmaftei
Am I confusing people, rather than clarifying?!
Cheers!
I got it, so it must not be TOO confusing.
doctorcisco
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03-04-01 12:27 AM
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Bernie
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Sorry, you confused me.
Poison Reverse is Split Horizon with Route Poisoning. Ex) ROUTERC IS THINKING...Hey, don't send packets back to RouterD, RouterD just told you it wasn't feeling good. I'd better tell RouterA and RouterB that RouterD has an infinite hop count.
By combining both it equals Poison Reverse?
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03-04-01 01:21 AM
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