Copied from cisco thread.
Preference is defined as follows:
(a) Intra-area and inter-area paths are always preferred over
AS external paths.
[OSPF]
(b) Type 1 external paths are always preferred over type 2
external paths. When all paths are type 2 external paths,
the paths with the smallest advertised type 2 metric are
always preferred.
[OSPF]
(c) If the new AS external path is still indistinguishable
from the current paths in N's routing table entry, and
RFC1583Compatibility is set to "disabled", select the
preferred paths based on the intra-AS paths to the
ASBR/forwarding addresses, as specified in Section 16.4.1.
Here intra-NSSA paths are equivalent to the intra-area
paths of non-backbone regular OSPF areas.
[NSSA]
Murphy Standards Track [Page 13]
RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003
(d) If the new AS external path is still indistinguishable
from the current paths in N's routing table entry, select
the preferred path based on a least cost comparison. Type
1 external paths are compared by looking at the sum of the
distance to the ASBR/forwarding addresses and the
advertised type 1 metric (X+Y). Type 2 external paths
advertising equal type 2 metrics are compared by looking
at the distance to the ASBR/forwarding addresses.
~[OSPF]
(e) If the current LSA is functionally the same as an
installed LSA (i.e., same destination, cost and non-zero
forwarding address) then apply the following priorities in
deciding which LSA is preferred:
1. A Type-7 LSA with the P-bit set.
2. A Type-5 LSA.
3. The LSA with the higher router ID.
So your N1 with the P-bit set should take precedence over an E1 if the metrics
are the same between the two!
RFC 3101 by the way.
From: [email protected]
To: [email protected]
Date: Fri, 26 Nov 2010 12:35:41 +0100
Subject: [OSL | CCIE_RS] anybody has final verdict on OSPF external routers
priority?
I’ve done some research about what is the priority order in the cisco routers,
regarding NSSA and regular external routes.
The problem is that it seems many IOS versions are still conforming with older
RFC1587, which is obsolete now because of the RFC3101.
Basically, the first one says E-routes are more preffered over N-routes, if
they are of the same type and with the same metric. The second one says the
opposite. :)
That is why labbing up such a behavior is not the answer. Does anybody know
from how, say, IOS 12.4 behaves? Or 12.4T maybe?
I would not like to load every single 12.4 IOS and try this out, you can
imagine that. :) If someone has the answer it will save a lot of time...
Best Regards,
Bojan Zivancevic
Network Engineer
----
Comutel d.o.o.
Omladinskih brigada 65v
11070 Belgrade
SERBIA
Tel: +381 11 217 8000 Ext.109
Mob: +381 64 646 8401
Fax: +381 11 6164641
http://www.comutel.co.rs
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