Subject: Re: [Isis-wg] draft-hermelin-ext-lsp-frags-00.txt

From: "Amir H." <[email-masked]>
Date: Tue, 16 Jan 2001 16:57:06 -0000

Jonathan,

the text implicitly prohibits the use of extended system-ids in pseudonode
LSPs. The reason is that permitting this would introduce zero-cost loops,
and this might upset some implementations. If the group decides using this
mechanism to overcome the 255 DIS limitation is worth the added risk ( IMHO
it is not ), I will add the text to support this.

I'll explain:
suppose system S has extended system-id V. In the current text, S never
advertises V in its hellos, hence adjacencies will only be formed with S,
not with V. This means (I will add this point to the text), that only S
advertises an adjacency to V. So, the SPF will always run by S first and V
later, hence the link from V to S can be advertised as non-zero, it doesn't
really matter what the cost is.
Now, if S advertises a pseudonode LSP under V, it needs to include this
pseudonode-id in its LAN hellos on that LAN. It must also include V as the
system ID on that LAN, to support previous implementations. So you'd have:
a -- S.01 -- S     V -- V.01 -- b
(a and b are systems on the two LANs).
So, since V can be reachable via systems other than S, you need a zero cost
adjacency between S and V in BOTH directions - hence the zero-cost loop.

--
Amir Hermelin   mailto:[email-masked]
Charlotte Networks LTD.   http://www.cwnt.com

----- Original Message -----
From: "Jonathan Sadler" <[email-masked]>
To: <[email-masked]>
Sent: Friday, January 12, 2001 4:17 PM
Subject: Re: [Isis-wg] New Internet-Draft

> Amir -
>
> Could the same form of extension be used to handle the limitation of 255
> L1 DIS interfaces per system?  I realize that this is less likely to
> occur than exceeding 256 fragments, but wording that supports this other
> situation may be appropriate to add to your draft.
>
> Jonathan Sadler
>

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