From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Sat Sep 27 2008 - 10:03:13 ART
> The interface used for the RID is the loopback (Lo0) interface on
> R3. I was just testing your suggestion (applying "no compatible
> rfc1583"), and SW4 still installs both routes towards 158.1.0.0/24.
RID actually doesn't come into it. I was forgetting that when the
forwarding address is 0.0.0.0, the path selected is the closet to the
ASBR, rather to the than the RID of the ASBR.
Both paths being installed in the RIB is explained by 16.4.1, just a bit
I overlooked :)
It appears to be because of the last line in the first paragraph below:
"
16.4.1. External path preferences
When multiple intra-AS paths are available to
ASBRs/forwarding addresses, the following rules indicate
which paths are preferred. These rules apply when the same
ASBR is reachable through multiple areas, or when trying to
decide which of several AS-external-LSAs should be
preferred. In the former case the paths all terminate at the
same ASBR, while in the latter the paths terminate at
separate ASBRs/forwarding addresses. In either case, each
path is represented by a separate routing table entry as
defined in Section 11.
This section only applies when RFC1583Compatibility is set
to "disabled".
The path preference rules, stated from highest to lowest
preference, are as follows. Note that as a result of these
rules, there may still be multiple paths of the highest
preference. In this case, the path to use must be determined
based on cost, as described in Section 16.4.
o Intra-area paths using non-backbone areas are always the
most preferred.
o The other paths, intra-area backbone paths and inter-
area paths, are of equal preference.
"
So multiple routes should be installed in the routing table if there are
multiple paths to the ASBR. Path preference will become an issue if
ospf is not permitted to install all of those paths.
Given the requirements of the question which you mentioned you can use
area 38 default-cost 10 on SW4 to stop the loop.
But just to complicate matters... I've just tried a simple setup (just
with 3 ospf 12.2 routers) and the behaviour I saw seems a little
different to the rfc:
- If the cost to the forwarding address via area 0 is lower than the
cost via area 38, then only the path via area 0 is installed in the
routing table.
- If the cost to the forwarding address via area 0 is equal to, or
higher than, the cost via area 38, then both paths are installed in the
routing table.
Paul.
Blogs and organic groups at http://www.ccie.net
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