Poison reverse example

Question

Can you give me an example when poison reverse is actually necessary?

Distance vector routing protocols employ split horizon with poison reverse to minimize the convergence time when a route is no longer available. The thing is that I can't think of an example when poison reverse is actually useful.

score 5 · Answer 1 · edited Oct 07 '21 at 07:34

The only useful example I could find for split-horizon/poison-reverse is multi-access routed segments (point-to-multipoint frame relay or an ethernet segment with >2 routers).

Both the RIP RFC (section 2.2.1), Cisco's EIGRP doc and Juniper's RIP document all show multi-access examples. Cisco's EIGRP RFC doesn't detail it's split horizon or poison-reverse implementation.

---Edit to remove previous incorrect info---

score 4 · Accepted Answer · answered Dec 29 '14 at 15:30

Consider the following topology:

               A
             / |
Internet -- S  |
             \ |
               B

Using RIP, S announces (0.0.0.0/0, 0), and both A and B announce (0.0.0.0/0, 1).

Suppose now that router S fails. Suppose further that you're unlucky, and that A and B both switch their next hops to each other -- they create a routing loop.

with plain Bellman-Ford, A and B have no way to get rid of the loop in a timely manner — they need to count to infinity;
with split horizon, A and B immediately stop announcing the default route to each other — they get rid of the loop as soon as the route times out;
with poison reverse, A and B announce an infinite metric default route to each other, which gets rid of the routing loop as soon as an update is successfully transmitted.

Note that poison reverse has a drawback — it increases the size of updates, sometimes dramatically so (especially from stub routers). Note further that poison reverse only gets rid of loops of size two — in order to get rid of larger loops in a timely manner, you need a feasibility condition, as in EIGRP or Babel.

Poison reverse example

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