Your second statement is the correct one.
Router A will load balance traffic destined for I to routers B and C in a
50% ratio.
Router A does not consider the two paths out of B to I for it's
load-blancing path.
Router A will only consider the number of equal next-hop's it has to get to
a destination when building the load-blanacing entries.
Router B will load balance the traffic it receives destined to I via D and
E. One caveat being that depending on the version of IOS you are running,
you may see all of the traffic from A destined to I passing through B
following only the link to D or to E, and not balanced between D and E. The
effect is called polarization and has changed so as not to happen in latter
versions of 12.0 and 12.1.
David
-----Original Message-----
From: eduard metz [mailto:e.t.metz@usa.net]
Sent: Wednesday, November 29, 2000 1:49 AM
To: cisco-nsp@puck.nether.net
Subject: [nsp] load-balancing problem ...
Hi all,
I've got a question on how Cisco routers load balance traffic.
Consider a network with 3 equal cost paths from A to I:
1.) A B D G I
2.) A B E G I
3.) A C F H I
Note that the first and second path share the same next-hop. Routing
protocol
is either OSPF or ISIS (link-state), CEF is enabled.
Will each path get (approximately) 1/3 of the traffic, i.e. link AB will bet
2/3 of the traffic and splits this further over BD and BE.
Or is traffic split by each node over the equal cost next-hops, i.e. A sends
50% over AB and 50% over AC, and B splits again on 50/50 basis, effectively
leading to 25% on paths 1 and 2, and 50% on path 3.
Would the loadbalancing behaviour by typical to the Cisco implementation, or
is this specific to the way link-state protocols work?
cheers, Eduard
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