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

From: "Amir H." <[email-masked]>
Date: Sun, 4 Feb 2001 19:24:29 -0000

(Sorry for switching the subject line, should've done it from the beginning)

Apparently, I was too hasty to dispose of the ECMP option. I don't think
that non-determinism is an issue, as RFC 1195 doesn't require any
determinism of IP paths greater than the MaxPathSplits. Even if some vendors
do employ determinism, that would not guarantee determinism in multi-vendor
environments. However, I agree that disposal of ECMP in the presence of
extended-LSPs is a bad idea. Especially since not only multiple paths to the
extended-LSP might be eliminated, but also paths to the original LSP (take
Naming's example, there is symmetry between B and B"). For anyone that's
confused, at least one path will still remain to all destinations.

The options I see as available right now are:
a) Don't advertise adjacencies in extended LSPs, as the original text
suggests. This has the advantage of avoiding zero-cost loops. This has the
dis-advantage of not being scalable in the presence of huge adjacency
information (over a thousand adjacencies advertising full TE information)
limited to only the original LSPs.
b) Advertise adjacencies off extended LSPs. This has the advantage of being
scalable to almost unlimited link-state information. However, it has the
strong dis-advantage of introducing zero-cost loops, and as a result loss of
equal-cost routes to some destinations.
c) Hybrid approach: use option (a) until the original LSPs have no more room
to hold adjacency information, then start advertising adjacencies off
extended LSPs. I don't like this one (since a single adjacency addition
could force a lot of ECMP paths to use just a single path), but it might be
better than the former two.

In any case, any of these options is better than not advertising the
information at all.

New options/opinions on these three are more than welcomed.

--
Amir Hermelin   mailto:[email-masked]
Charlotte Networks Inc.   http://www.cwnt.com
In Simplicity there is Elegance(TM)

Tel: 972-4-9592203 ext. 222      Fax: 972-4-9593325
2 Ha'Mada St., POB 333, Yokneam Moshava 20600, ISRAEL.
----- Original Message -----
From: "Tony Li" <[email-masked]>
To: "mike shand" <[email-masked]>
Cc: "Naiming Shen" <[email-masked]>; "Amir H." <[email-masked]>; "Tony
Li" <[email-masked]>; "ISIS-wg" <[email-masked]>
Sent: Thursday, January 25, 2001 6:03 PM
Subject: Re: [Isis-wg] Re: New Internet-Draft

>
>  | I imagine that we could modify the SPF algorithm to deal with this
(e.g. by
>  | selectively permitting additions to the adjacency set of nodes already
on
>  | PATHS, or by unwrapping the zero cost links onto TENT, while keeping
track
>  | of possible loops), but that is not the point. The whole rationale for
this
>  | proposal is that we can allow certain nodes to expand their LSP set
WITHOUT
>  | requiring any changes in the other nodes, and in particular any changes
to
>  | their SPFs.
>  |
>  | So I guess the question is, do we care about the failure to make use of
>  | potential path splits? I think we do.
>
>
> I concur.  And I agree that a solution that required no changes to
existing
> systems would be preferable.  Any suggestions?
>
> Tony

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