[rbridge] Access-only link

[Radia Perlman]

I forget who mentioned to me at the last IETF the desire to have an 
"access only" port. At the time I found it
scary.

However, after thinking about it, I've convinced myself it's not scary, 
and relatively simple to provide.

We've already added in "trunk only" links, on which data is not 
encapsulated/decapsulated if there happened to
be endnodes present despite configuration to specify the link as "trunk 
only". That feature seemed simple to
provide and useful to me.

I'm a little less clear on why access-only links are as important, since 
it doesn't seem as though the traffic of
LSP propagation would be that much. But again, I think it's simple and 
safe, so if people think it's useful, I wouldn't
oppose it.

The idea is for the DRB to signal in its Hello "this is access-only". 
And in the LSP for the pseudonode, the DRB
can set the "database overflow" flag, which is a flag already in IS-IS 
for the purpose of dealing with a router that
can't hold the LSP database. That flag means "don't go through this node 
unless absolutely no other path exists".
If the "overflow" flag is set in the pseudonode's LSP, that would mean 
that that pseudonode would not be computed
in a path as a transit link.

Radia