[rbridge] per-VLAN instances of IS-IS

[Anoop Ghanwani]

Section 3.3.3.2 of the rBridge base spec 
talks about the per VLAN instance of IS-IS.

I can see why that would be useful.
However, if looks like there could be
problems scaling.  If an end rBridge 
participates in 1000 VLANs, it will have
to maintain 1000 adjacencies.  

Plus, it looks like for the per-VLAN IS-IS
instance, the RBridge network operates
as single LAN segment.  That would mean
having to go through DR/BDR election for
each instance.  

Operating 100's of routers on a single 
LAN segment would poses its own set of 
scaling issues.  Having per VLAN instances
exacerbates the problem.

Have the scaling issues been considered
at all?  It might actually be better to
just use the core IS-IS instance.

The following statement in Section 3.3.3.2
of the draft appears to be incorrect/incomplete:

"RBridges with endnodes in VLAN A also 
receive and process the frame in their 
VLAN-A IS-IS instance."

The frame also has to be processed by all
RBridges that might be along the path for
reaching the VLANs, otherwise we wouldn't
be able to prune the distribution trees
correctly.

Anoop