[rbridge] encoding of TRILL IS-IS frames

[Radia Perlman]

Someone asked me why IS-IS frames need to be TRILL-encapsulated, and I 
don't remember. I do know we
were somewhat concerned about differentiating TRILL IS-IS frames from 
layer 3 IS-IS frames. But assuming
we use a different multicast address for TRILL IS-IS than layer 3 IS-IS, 
with the additional safeguard of
using a different "area ID" (for TRILL, area ID=0), it seems like there 
would be no danger of confusion.

Perhaps we were concerned about possible unicast of IS-IS frames, like 
for instance, a PSNP that one might
send just to the DR? I think I verified with the IS-IS people that there 
are no IS-IS frames that are unicast.

So why doesn't the following work?

For core IS-IS frames, no encapsulation, but a special multicast address 
"all-nbr-TRILL-IS-IS" as
the destination address.

For ESADI, TRILL encapsulation like with ordinary data packets, but 
having as the destination
address in the inner Ethernet header a different multicast address 
"all-ESADI-TRILL"

The advantage of this is that for core TRILL IS-IS, we'd save header 
room and probably work for
RBridges to parse IS-IS packets.

Radia