Subject: [Isis-wg] Route selection

From: Neal Castagnoli <[email-masked]>
Date: Wed, 22 Aug 2001 13:18:10 -0700

We ran a battery of route selection tests against a couple of common
implementations.  One of the implementations in recent versions follows the
RFC 2966 route selection precedence, but in older (deployed) versions treats
the metrics as 8 bits wide.  The other treats external metric, level 2, and
external route (TLV 130) at an absolute lower precedence than L1 internal
routes, regardless of the disposition of the up/down bit.

Given the inconsistent way these common implementations perform route
selection and the attendent routing loops that can form, does it make sense
to avoid use of external routes (TLV 130) altogether?  Does anyone know of
deployed instances in which the use of external routes is required?

-----------------  Example:

Routing loop caused by inconsistent route selection.  C1 is an
implementation that absolutely prefers L1 internal routes, C2 chooses based
on least cost.

                  10       10    10
               A  ---- C1 --- C2 --- B

If A advertises a.b.c.d as an external route of cost 10, but B advertises
the same route as internal at cost 30, then C2 will use the cheaper route to
a.b.c.d through C1 to A at a total cost of 30, but C1 will prefer the more
costly route through C2 to B at 50 since it prefers internal routes to
external routes regardless of their cost.
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