Subject: Re: [Isis-wg] Computing hop by hop routes with wide (TE Draft) metrics

From: stefano previdi <[email-masked]>
Date: Thu, 11 Jan 2001 18:34:49 +0100

At 23:40 10/01/2001, Neal Castagnoli wrote:

>In reviewing the ISIS TE draft, I'm wondering how to mix wide (TE draft) and
>narrow (original) links and routes.
>
>One way to build a migration path would be to run two SPF computations, once
>on each metric space (using only "narrow" or "wide" metrics in each run).
>This method has the disadvantage that during migration wildly suboptimal
>routes can occur, because during migration computations only use a subset of
>the links.
>
>Other options include administrative control. For example:
>
>New routers compute using both "old" and/or "new" metrics.  New routers can
>have paths composed of links with old and new metrics in it.
>
>Start:
>   . all old routers, only reporting old metric
>   . one by one change to new routers, also reporting only old metrics.

I would suggest: "New routers report BOTH old and new metric"
(using same value of course).

>   . When all routers are "new", reconfigure each router so it repots only
>new metrics on each of its links.
>
>This method has disadvantages, including that the administrator needs to be
>extremely careful with the router versions and that routes can also be
>suboptimal.

New routers should report adjacencies and IP prefixes in both 
TLVs (narrow and wide) and the metric value must be the same. 

Then, whatever is the TLV you use for your SPF, you'll get 
consistent SPTs. New routers will be able to consider both 
old and new metric but this doesn't hurt SPF as long as the 
metric value is the same.

s.

>I don't think either of these are great solutions.  Perhaps someone would
>like to suggest others.  But at any rate, it seems that how to compute hop
>by hop routes needs to be specified as a part of the TE document.
>
>  --Neal
>
>P.S. Sorry if this gets duplicated.
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---

From: "Amir H." <[email-masked]>
Date: Wed, 17 Jan 2001 15:18:00 -0000

> >by hop routes needs to be specified as a part of the TE document.

I agree with Stefano that the advertised metrics should be the same in both
old and new-style TLVs; however, at least one dominant implementation
doesn't always advertise it that way. In that case, Tony's mention of an
algorithm that allows only paths that don't go from narrow to wide metrics
(or "old" to "new" routers) would intuitively install only loop free routes.

In any case, Neal has a good point that the TE document should mention this
in a clear way. Specifically, the TE document should define:
- What is advertised in case of a metric greater than 63 in transition mode
(both metrics being advertised).
- What should be the SPF procedure, in case corrective measures need to be
taken? As Stefano pointed out, there is only need for special SPF in case
some links differ in their narrow and wide metrics. There is also a problem
if a link advertises only wide metrics, but that's more of a
misconfiguration than a protocol problem.

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

---

From: Tony Przygienda <[email-masked]>
Date: Wed, 17 Jan 2001 19:16:01 +0100 (MET)

> doesn't always advertise it that way. 

my initial reaction would be they rather revisit and fix unless there is a 
reason to make mertics non-matching which I fail to see right now.

> misconfiguration than a protocol problem.

there is a distinction between a specification and deployment and transition
issues so if you want that documented I'd rather think about another draft.
TE draft will be pretty timeless (= ~5 years in Internet) whereas the migration
issue if any will be a month's flavor. 

	-- tony