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

From: Radia Perlman - Boston Center for Networking <[email-masked]>
Date: Sun, 17 Oct 1999 22:29:52 -0400 (EDT)

OK, Here's some proposed text for adding the ability to be DIS
on more than 255 broadcast links. If you're actually editing it,
I'll mention that "More trickier" isn't proper English. (I wouldn't
have bothered except if you're editing anyway...) You should say
"A tricker case".

Anyway here is proposed text for the
>255 LANs. I think the easiest way to shoehorn it in without
too much editing is to add a new section called:

Being DIS on more than 255 broadcast links

and have the section that says it's "out of scope to discuss this"
to point to that section. And the new section should say something like:

Being DIS on more than 255 LANs

It is possible to be DIS on more than 255 LANs provided that a 7-byte
pseudonode ID, unique in the area, can be selected. The IS-IS
specification states that the DIS's system ID is the first 6 bytes of
the pseudonode ID. However, the only property of the 7-byte Pnode ID
necessary for correct operation of the routing algorithm is that
the 7-byte ID be unique in the area.

If router R has multiple MAC addresses, say MAC1, MAC2, MAC3, then R
can easily derive 255*3 unique Pnode addresses, since it can use
MAC1.1, MAC1.2, ... MAC1.255, MAC2.1, ... MAC2.255, MAC3.1, ... MAC3.255.
A router that might be DIS on more than 255 broadcast links can also
be configured with multiple EUI-48 addresses for this purpose, even
if they are not used as layer 2 addresses.
If R only has a single EUI-48 address, it can still
generate 4 unique 6-byte prefixes for Pnode IDs, because it can use
the global/local bit and the group/individual bit in its EUI-48. In
other words, it can use any setting of those 2 bits in its EUI-48
address for this purpose.

Note that although there is no reason why the first 6 bytes of the Pnode
ID must be the system ID of the DIS, it is true that the IS-IS specification
does say that this will be the case, and although we know of no problems
that this will cause, it is possible that some management tools might
rely on that assumption.

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