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Home > Archive > CCNA > August 2001 > Split Horizon Vs Poison Reverse
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Split Horizon Vs Poison Reverse
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| HOOLIGAN 2001-08-28, 5:12 am |
| I understand what they both do, but I fail to see the advantage of Poison reverse?
Cheers in advance.
Hooligan | |
| dmaftei 2001-08-28, 7:10 am |
| The convergence time is shorter with poison reverse. | |
| Gobolla 2001-08-28, 9:15 am |
| Well,I have following opinion:
Split horizon prevents a router interface from advertising a route back to it's source.
While
In Poison reverse route updates are sent back to source but the routes learned via source are marked as poisoned.
Ok
So,What is the advantage of poison reverse:
Obvious!
"A router can advertise routes,it has learned from other source routers."
e.g.
Consider the following internetwork:
RouterA---------RouterB--------------RouterC
If split horizon is enabled b/w R-B & R-C,What is the disadvantage,R-B cann,t advertise routes learned via R-A back to R-C.
However,if we implement poison reverse instead of split horizon,whats the advantage,Obvious! R-B can now advertise routes learned from R-C => R-A and from
R-C => R-A.
Thanks! | |
| depamo 2001-08-29, 3:35 pm |
| Split Horizon and Poision Reverse are both bandaids for problems found after certain routing protocols were made. Split Horizon is an obvious issue, don't repeat a learned route out the same port you heard about it on, but posion reverse is a little different.
Depending on the radius of your network with a Distance Vector Protocol (like RIP), when a change in the network occurs, a route drops, the first router to learn about the down route will 'poision' the route so all routers that have paths using this route will not attempt to use then until the network converges again. At the convergence point, the route is removed from the routing table. The whole trick is to keep all routes that use that hop from advertising alternate or using this route until the network has time to converge and flush the routes from the table.
This is in conjunction with Hold-Down Timers are why Distance Vector protocols take so long to converge. Route goes bad, poision it, activate hold-down timer on route, advertise poisioned route. |
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