[rbridge] I got some answers from the IS-IS mailing list

[Radia Perlman]

Oh definitely. We need both (putting 0 into the area address, and either 
a multicast
address or a bit in the header).

Radia


Dinesh G Dutt wrote:
> Hi Radia,
>
> Radia Perlman wrote:
>> 1) They have deployed a variant of IS-IS, and the way they 
>> distinguish their instances
>> is based on having a new multicast address. I wrongly remembered 
>> IS-IS, and thought that
>> PSNPs got unicast, but even those are multicast (and I remember the 
>> reasoning at the
>> time this was decided 100 years ago, which was to suppress lots of 
>> routers sending the
>> same PSNP). So, for encoding TRILL IS-IS, it can be differentiated 
>> from layer 3 IS-IS
>> by having a different multicast address. The bit in the header works 
>> too, however.
>>
>> 2) I checked to make sure that "0" would be OK to use as an area 
>> address, and it is.
>>   
> My reading of what they recommended is that we use a different 
> multicast address since it was not guaranteed that all implementations 
> would support a 0 in the area address.
>
> Here is a pointer to the email from Mike Shand: 
> http://www1.ietf.org/mail-archive/web/isis-wg/current/msg01746.html
>
> Dinesh
>