[rbridge] Ambition with different VLAN configurations

[Erik Nordmark]

The list has had discussions on how to do hellos with VLANs, and the 
discussion makes me wonder whether we have agreement on the level of 
ambition with various unusual VLAN configuration in the bridged LANs.

By "ambition" I mean whether in a particular case we should:
1. Make RBridges provide full connectivity for all the nodes in the 
bridged LAN
2. Make RBridges at least detect that there is something odd about the 
VLAN configuration and ensure that no permanent loops can be formed. But 
it would be ok to orphan some nodes from communicating.

One can ask that question for different VLAN scenarios.
One interesting case that Donald brought up is when there isn't a single 
VLAN tag on which all the RBridges can communicate.
For instance, RB0 and RB1 can only send and receive using VLAN tag 3.
RB2 can only send and receive using VLAN tag 4.
RB3 and RB4 can send and receive using both VLAN tag 3 and 4.

There are different schemes for making hellos detect the connectivity in 
this case. So let's ignore that and instead look at the implications on 
the data plane if we want ambition #1.

Suppose RB4 has a multicast to send to all RBridges. Which VLAN tag 
should it use? There isn't a single VLAN tag that it can use whichever 
reach all RBridge neighbors. But RB4 can't treat the port as two 
logically different ports (one for VLAN tag 3 and a separate one for 4) 
either, since RB2 is a neighbor for both VLANs.
Thus if we wanted this case to provide full connectivity it would seem 
like the data plane would need to be able to duplicate packets and send 
some unicast; a possible strategy would be to multicast with tag 3, and 
then unicast a copy to RB2 (with tag 4). That way all the neighboring 
RBridges would receive the frame.

That sounds complex.

There might be other examples of odd VLAN configuration (such as bridges 
which untag and then add some other tag when forwarding the frame) which 
are complex to fully support. What level of ambition should we have for 
the odd cases?

    Erik