From: Gavin Lawson (GavinL@titan.net.au)
Date: Wed Jan 03 2007 - 02:04:58 ART
OSPF will select routes in the following order.
. Intra area route.
. Inter area route.
. External route.
The cost is look at after the area identifier.
So if an external route has a lower cost than an intra area route - the
intra area route will still be selected.
GL
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Jian Gu
Sent: Wednesday, 3 January 2007 11:33 AM
To: Cisco certification
Subject: OSPF path selection
Hi, all,
If a router receives an external route with type 1 metric coming from
intra-area (higher cost) and inter area lower cost from the same
originator, how OSPF selects the routes? I am having hard time to grasp
how RFC says in this scenario.
********* RFC2328 ***************
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.
************************************************************************
********
I don't quite understand what second bullet means, does it sugguest that
OSPF will loadbalance between intra-area backbone paths and inter-area
paths?
Or it is totally implementation dependent?
Thanks a lot,
Jian
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