Re: [rbridge] Call for draft-tissa-trill-cmt-00 to WG draft

[Radia Perlman <[e-mail]>]

It is my understanding that
a) CMT does require changing all the RBs in the campus
b) CMT requires there be at least as many trees as there are active/active
RBs on any link

Of course, no solution is ideal.  It might have been nice to write up all
the proposed solutions to this and pros/cons. Maybe it's
still worth doing.

So from memory...other proposals for allowing R1, R2, and R3 to be
active/active/active on a link:

First note: regardless of whether their port to the link is in a tree,
unicast works, and they can use the pseudonode
nickname when encapsulating unicast.

Also, if Ri's port to the link is in at least one tree, Ri can use the
pseudonode nickname for that link when encapsulating
multidestination frames for ingress, and the RPF check will work without
any problems.

However, if Ri's port to the link is not in any of the trees,
here were some of the proposed solutions.  So, let's say that R1 and R2's
port to the shared link is in at least one tree, and R3's
port to the link is not in any of the trees, and R3 needs to encapsulate a
multidestination frame:

1) R3 could tunnel it to one of {R1, R2}, and let them inject the packet
(with pseudonode nickname) into the campus.

Pro: Doesn't affect any RBs other than the ones on the link; backwards
compatible.  Con: more hops, might be difficult
in some implementations for R3 to forward by tunneling, might be difficult
for R2 to accept a tunneled packet.

2) R3 could use its own nickname instead of the pseudonode nickname in this
case

Pro: Doesn't affect any RBs other than the ones on the link; backwards
compatible.  Con: MAC learning in distant RBs will have
frequent learning changes of the source MAC S between being on the
pseudonode nickname (whenever S sends unicast,
or multicast through R1 or R2), or being on R3 (when R3 has to encapsulate
a multidestination frame from S).

------
There might have been some other proposals, but they wound up not to work.

Personally, I don't love the CMT thing because of the two disadvantages I
mentioned above, but I can live with it.

Radia

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