[Isis-wg] Re: internet-drafts/draft-ietf-isis-wg-255adj-02.txt

[Radia Perlman - Boston Center for Networking]

>> When using this technique a node can use arbitrary number of circuits
>>    only being restricted by the fact that it cannot be DIS on more
>>    than 255 circuits since PNodes would become indistinguishable for
>>    different LANs.  To solve that problem, different techniques, such
>>    as using multiple router IDs would be necessary, are however outside
>>    the scope of this draft. 

It really is quite simple to use multiple router IDs and then you don't
have the restriction preventing being DIS on more than 255 circuits.
I mentioned this in email before. A router will usually have multiple MAC
addresses, and certainly it can be given multiple addresses if it's
attached to more than 256 LANs. The only restriciton on the 7-byte LAN
ID is that it be unique within the area, so if the router has MAC addresses
A1, A2, and A3, then it can use
A1.1, A1.2, ...A1.255, A2.1, A2.2, ... A2.255, A3.1, etc.

There also isn't any reason not to play with the "global/local" and
"group/individual" bits in the OUI, so even if a router only had
1 MAC address, it could form 4 different 6-byte addresses that no other
routers in the area would own. Again, the only thing necessary for
proper operation of IS-IS as a routing protocol is that the 7-byte
LAN ID be unique in the area. There's no necessity for the first 6
bytes to be the same as the ID of a router on the LAN...it was just
that using a router's 6-byte
ID as a prefix was a convenient way to obtain a unique LAN name.

Radia