RE: preferring OSPF inter-area compared to intra-area

You can use distance to filter, but you can't use it to prefer one route type over another. If you have an Intra-Area route and an Inter-Area route for the same prefix, the Intra-Area route is chosen as best and goes to the OSPF RIB. The distance is then applied to the OSPF RIB before the route goes to the global RIB, which is the routing table. The issue becomes that even if you poison the Intra-Area route by setting its distance to 255, it was the only route in the OSPF RIB to begin with, and the end result is that OSPF feeds no route to the RIB for that prefix.

In other words you can deny OSPF from installing the prefix in the routing table overall, but you can't deny it from installing the Intra-Area route in order to get it to install the Inter-Area route instead.

This is the same type of reason that Sham Links are implemented in MPLS L3VPN with OSPF as the PE-CE routing protocol, because if you're migrating off of a private layer 2 VPN service like Frame Relay to MPLS L3VPN, your Intra-Area routes over the Frame Relay network would always be preferred over the Inter-Area or External routes coming from the MPLS L3VPN. The fix is to run a Sham Link over the MPLS L3VPN which makes the routes be able to come in as Intra-Area. If you now have an Intra-Area route over Frame Relay and an Intra-Area route over MPLS you can just change the link costs so that the Frame Relay is used as a backup.

Brian McGahan, CCIE #8593 (R&S/SP/Security)
bmcgahan_at_INE.com

Internetwork Expert, Inc.
http://www.INE.com

-----Original Message-----
From: marc edwards [mailto:renorider_at_gmail.com]
Sent: Tuesday, January 08, 2013 4:45 PM
To: ccie99999
Cc: rakesh madupu; Jay McMickle; Brian McGahan; Narbik Kocharians; Carlos G Mendioroz; Cisco certification
Subject: Re: preferring OSPF inter-area compared to intra-area

Try using distance command under ospf process of R6 to manipulate path selection. I am surprised this wasn't suggested..... or did i miss it?

Why don't you try using distance command under ospf process

On Tue, Jan 8, 2013 at 12:19 PM, ccie99999 <ccie99999_at_gmail.com> wrote:
> Hi Rakesh,
>
> yeah.. I'm just filtering rib and not ospf db.
> I've no idea of I could make an inter-area route become an intra-area,
> using a route-map or a filter.
> Best way is move the interface to the same area or use a VL I need to
> lab it a bit.
>
> On Tue, Jan 8, 2013 at 5:19 PM, rakesh madupu <raaki.88_at_gmail.com> wrote:
>
>> Rakesh
>
>
>
>
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Received on Tue Jan 08 2013 - 17:45:09 ART