Subject: Comment on ISIS extensions for traffic engineering

From: Jeff Learman <[email-masked]>
Date: Fri, 23 Jul 1999 13:47:32 -0400

The IS-IS extensions for Traffic Engineering

( http://www.ietf.org/internet-drafts/draft-ietf-isis-traffic-01.txt )

sets a new value for MAX_PATH_METRIC.  I have two reservations
about this.

First, it will not be interoperable with previous implementations,
if any paths in the network exceed the old value for MAX_PATH_METRIC
(1024), because different routers may choose conflicting paths,
thus causing persistent routing loops.

Second, the low value of MAX_PATH_METRIC was chosen to permit an
optimization in SPF calculation that reduced it from O(NlogN) to
approach O(N).  Has the processing power on routers outpaced the
growth in subdomain size sufficiently that this optimization is
no longer necessary?

Regards,
Jeff

____________________________________________________________________

  Jeff Learman                           Internet:     [email-masked]
  Engineering Manager                    Voice:     (734)-975-7323
  ONE (Open Networks Engineering, Inc)   Fax:       (734)-975-6940
  2725 South Industrial Pkwy, Suite 100
  Ann Arbor, MI  48104  USA
____________________________________________________________________

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From: Henk Smit <[email-masked]>
Date: Mon, 26 Jul 1999 23:39:23 +0200 (MET DST)

> First, it will not be interoperable with previous implementations,

  This correct. Nobody said it would.

> thus causing persistent routing loops.

  The set of old-style TLVs and the set of new-style TLVs are
 not freely mixable. You need to upgrade and transition whole
 areas at a time, or the whole L2 domain at a time. There are
 strategies to do gradual transition without a flag day, but
 we decided that transition is outside the scope of the document.

  Note that draft-ietf-isis-traffic-01.txt defines a new TLV for
 adjacencies. Older implementations that do not understand this
 new TLV will miss out on a lot of links in topology. And thus
 they will have a whole different view on the network as the
 routers with the new extensions. You can't mix them.

> growth in subdomain size sufficiently that this optimization is

  I would say yes.
 Of course customers or vendors that worry about the CPU utilization
 for their SPF computations are free to not implement the new
 extensions, and thus do not need to support an new sorting algorithm
 for the tentative list.

     Hope this helps,

           Henk.