[rbridge] Orphaned endnodes with partitioned VLANs on a cloud (Example 1)

[Russ White]

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Suppose we have a simple network, where A, B, and C are all connected to
a single link, a broadcast of some type:

A--+
   |
   +--B
   |
C--+

Now, assume all three are members of VLANs 1, 2, and 3 on this link. All
three are properly configured to use VLAN 1 as the rbridge vlans. A is
designated as the DRB.

Now, things are running along fine, but, for some reason, A
malfunctions, or some such, and can no longer send multicast packets on
VLAN 1. The result is that B and C detect they've lost their connection
to A, but they won't elect a new DRB at all. Why? Because they are still
receiving hello's with A claiming to be the DRB, and with A's original
priority and ID, on the other two VLANs.

Now the link is completely out of the SPF tree, and not used at
all--because of a partial failure on the DRB. This is broken, and
unexpected behavior.

The IS-IS hellos are there to find per link bidirectional reachability.
When you take that away, and assume reachability on one logical link
because you have reachability on another logical link, you wind up in a
mess that's harder to fix than the original problem you were trying to fix.

:-)

Russ

- --
riw at cisco.com CCIE <>< Grace Alone

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