[rbridge] Updating input fliters before output filters after a toplogy change

[Radia Perlman]

Dinesh pointed out that temporary loops are less likely to be formed for 
multidestination
traffic if after a topology change RBridge R updates its input filtering 
tables on all its ports for all trees
before updating its output filtering tables.

Note by "input filtering" I mean the RPF (reverse path forwarding) check 
that it makes sense to receive
a packet from that ingress RBridge on that port for that tree. In other 
words, for each port, and for each tree,
it is the set of ingress RBridges that traffic on that tree can be 
received from, on that port.

By "output filtering", I mean the set of output ports in that tree, 
along with the set of VLANs, multicast
addresses, and reachability of IP routers, etc., on that branch.

Intuitively it seems safer to have your first action
after a topology change be to stop receiving on ports that are no longer 
in the tree.
If everyone does this, then being slower about updating which ports you 
output to will be less problematic.

Anyway, it also seems safer if all RBridges update in the same order.

So I think it's a good idea to say in the spec that the input filters 
MUST be updated before the output filters.

Radia