From: ccie2be (ccie2be@nyc.rr.com)
Date: Sat Jul 19 2003 - 00:44:04 GMT-3
Hey Banlan,
There isn't a simple yes or no answer to your question. However, I can give
you some things to think about in order to answer your question.
Let's suppose you have 3 rtr, r1, r2 and r3 connected via F/R in a partial
mesh with r2 the hub router. For r2 which learns routes from r1 to
advertise those same routes to r3, it needs split horizon to be off. If
split horizon isn't off, r2 can't advertise the routes it learns from r1 to
r3 because it can't advertise routes it learns from an interface out the
same interface. Therefore, on r2 split horizon must be turned off or else
r1 will not learn of r3 routes and vice versa.
But, you need to think about this: if r2 is advertising routes it learns
from r1 to r3 when split horizon is turned off, won't it also be
advertising, those same routes back to r1? I'm pretty sure the answer is
yes in which case r1 may incorrectly believe r2 is the next hop router for
routes that r1 learned (let's suppose) from anther router, say r4. If this
happened, that would be a very bad thing. Who should r1 believe is the next
hop router, r2 or r4 for a route that r1 learned from both r2 and r4?
Actually, it may not be the end of the world - and I forget some of the
details - because when r1 learns the same route from both r4 and r2, the
routes from r4 will have a lower hop count. If I remember correctly though,
things can get dicey if a route learned from r4 goes down. Check out
Doyle's Routing TCP/IP for a more complete explanation. But, in any case,
you can avoid r1 having any confusion by filtering the routes learned from
r4 on their way in from r2.
So. hopefully, it's clear that on the hub router in a hub and spoke F/R
partial mesh, the hub router must have split horizon disabled. This is true
when the f/r interface on the hub router is either the physical interface
itself or a point to multipoint subinterface. If it's a f/r p2p
subinterface then split horizon should be turned on (or left on).
BTW, I can never remember the default setting for split horizon on a f/r
interface or subinterface. But, that's not so bad as long as I remember
that it's easy to check. Just do a show ip interface for the interface
running f/r and the output will tell you if split horizon is enabled or
disabled.
Now, what about the spoke routers. If r1 learns routes from r2 on it's f/r
interface should r1 advertise those same routes back out to r2? Probably
not. therefore, on the spoke routers I would make sure split horizon is
enabled.
I'm reasonably confident of the correctness of what I've told you.
Hopefully, if I misspoke, someone on group study will quickly catch and
correct any that i've said that's not 100% correct.
raj
----- Original Message -----
From: "Banlan Chen" <banlan.chen@lycos.com>
To: <ccielab@groupstudy.com>
Sent: Friday, July 18, 2003 10:19 PM
Subject: Question about Split-Horizon on NBMA
> Hi Group,
>
> When I configure RIP over two FR main interfaces, I should enable
split-horizon on each interface or just enable it at one of these
interfaces.
> Under ATM interface, Do I need configure ip split-horizon for RIP and
EIGRP?
> Thanks in advance.
>
> Banlan
>
>
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