Subject: [rbridge] parallel links

From: "Jeff Pickering" <[email-masked]>
Date: Fri, 10 Apr 2009 05:46:07 -0700

New to the list and was looking at a couple of threads related to parallel links. If I missed something
in another  thread that addressed this, I apologize in advance..

For the case where R1,R2 have parallel links, it seems there is agreement that a third device R3
need not be concerned. Ie, R3 need only know whether or not the shotest path transits R1/R2. So
it seems to me there is no need to suppress links in R1 or R2's LSPs and no need to worry about
unequal costs reported on those links.

Jeff

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From: Donald Eastlake <[email-masked]>
Date: Fri, 10 Apr 2009 18:00:26 -0400

If different RBridges compute shortest paths in different ways, you
can get loops. So, given a link state database, there must be a
deterministic way of doing this.

The current (-12) draft requires that multiple parallel adjacencies be
reported as a single adjacency in LSPs and that if, in violation of
that requirement, they are reported separately, you just use the
lowest cost adjacency (or one of the lowest cost if there are two or
more tied). The first requirement could be dropped, given the second
provision. But whatever you do, you have to avoid cases where two
different RBridges, operating on the link state database, would come
up with different costs for the adjacency, such as one using the cost
of the least cost adjacency and another assuming that their was some
equal cost multi-path going on or the like and assuming that
additional parallel adjacencies decrease the cost of the hop.

Donald

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