[rbridge] Pseudonode minimization thoughts...

[Radia Perlman]

The WG seemed to think that the pseudonode minimization was a good thing 
for all link state protocols, and therefore
should be proposed in the routing working group rather than in TRILL.

I'm not convinced it is possible to put in the extra flags necessary for 
OSPF, so perhaps it should just be presented in IS-IS instead.

Also, I was thinking about a rationale for a good cutover. What I 
proposed originally, picking numbers out of
the air, was 1 or 2 routers, no pseudonode, 5 or more, pseudonode, and 
anything in between, stick with what it was.

Originally, IS-IS was designed for CLNP, and it was necessary to report, 
for each link, all the attached endnodes. So
even if there were only 2 routers on a LAN, it made sense to create a 
pseudonode, so that all the endnodes wouldn't
get reported by both routers.

But for TRILL--there really isn't anything reported in the pseudonode 
other than the set of router neighbors.
Things like the set of supported VLANs, and the set of roots that an 
RBridge might select for a multicast tree, are all
reported in the RBridge's individual LSP. The only thing in the 
pseudonode LSP is the set of RBridge neighbors.

So, I'm thinking that the right cutover would be something like 15 
RBridge neighbors before it's worth creating another
LSP that has a whole header, and appears in every other RBridge's CSNP.

I  must say it's not easy to find the packet formats for IS-IS to get 
the exact number of bytes in an LSP. :-(

Radia