[rbridge] Can we ever have pt-to-pt links?

[Radia Perlman]

FYI: This is the proposal I'm running by the IS-IS list. Summary: DRB 
chooses whether
or not to create a pseudonode on the port, and tells the other RBridges 
whether to
claim pt-to-pt links to the other RBridges on the link, or attachment to 
a pseudonode.
Algorithm: if R1 has zero or one RBridge neighbor, no pseudonode. If R1 has
4 or more RBridge neighbors, R1 creates pseudonode for the port. If R1 
has 2 o3
3 RBridge neighbors, R1 leaves the state of the link (pseudonode or not) 
alone.

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a) perform normal DR election. If R1 is the only router, R1 is the DR. 
Assuming there
are other routers on the link, say R1, R2, R3, and R4, let's say R1 is 
the DR.

b) R1 (DR) is the one that decides on behalf of the link whether there will
be a pseudonode or not.
R1 has some algorithm for deciding if there should be a pseudonode. In 
R1's Hello
it says "the name of the pseudonode is R1.x", which means that there 
will be a pseudonode.
The other routers issue LSPs claiming to be connected to R1.x. 
However....if R1
decides not to have a pseudonode, then R1's Hello says "the name of the 
pseudonode is 0"
in which case any routers on the link claim the other routers on the 
link as direct neighbors.

c) a reasonable strategy for DR R1 to decide when to have a pseudonode: 
If R1 has zero or
one router neighbors on the link, R1 puts the link into "no pseudonode" 
mode.
If R1 has at least 4 router neighbors, R1 puts the link into 
"pseudonode" mode.
If R1 has 2 or 3 router neighbors, R1 does not change the state of the 
link. (so 3 router
neighbors have to come up or go down before R1 switches the link between 
pseudonode
vs no pseudonode).

d) if you aren't DR, then you do as you are told.

I believe this proposal is
1) zero configuration
2) safe
3) simple
4) not disruptive if a router is going up and down
5) no danger of confusion
6) much more efficient than creating a pseudonode for every port