[rbridge] Setting initial "hop count" value
Radia.Perlman at sun.com
Wed May 2 09:18:50 PDT 2007
We didn't say what the initial value of the "hop count" field should be.
One tempting thing would be to set it to the computing number of hops to
the egress RBridge, since the ingress RBridge knows how long the path is
computation from the link state database). It would be easy for an
include not just the next hop to the egress RBridge, but the hop count
so that it would know how to set the hop count field.
However, layer 3 people these days do various clever "detour" things
can't reach the next hop, so that packets can still get to the
the routing algorithm is converging. So the hop count could need to be
what the ingress guy thinks.
But I think it's a good idea for multicast for the ingress RBridge to
set the initial
hop count to be equal to the maximum length from that RBridge to any
(in the selected tree).
We even could be clever and an RBridge in the core that is forwarding on
could decrement the hop count by more if it knows that some branch of
distribution tree needs a smaller count in order to reach to the ends of
This would make multicast even safer.
At any rate, it's always seemed a bit weird to have a field like "TTL"
in IP without
saying in the spec what to set it to. We don't have to make the
suggestion a MUST, but it would
be nice to give some recommendation in the TRILL spec about what to set
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