[rbridge] Rbridge port security

[Eastlake III Donald-LDE008]

In connection with the topics in this thread:

I've looked at IS-IS security some more and the more recent versions of
it seem to provide strong protection against forged IS-IS hellos or
other control traffic. This generally seems to be an existing and better
way of handling this problem than my original suggestion of "turning
off" IS-IS on a port.

Caitlin's suggestion that TRILL encapsulated frames be ignored when
received on ports on which there is not an Rbridge adjacency is a good
one and it could be expanded a little to also drop such frames if their
source MAC address isn't that of a known Rbridge.

Thanks,
Donald