OSPF route selection

[dale.cooke]

Hi we are currently converting our network from Frame-relay/OSFP to MPLS with e-bgp. We are redistributing the MPLS e-bgp process

into OSPF. We want to leave the OSPF frame network in place as a backup for the time being. The problem I face is that the OSPF routes

seem to be prefered to the redistributes BGP routes even if I change the admistrative distance for external ospf routes to 90.

The two routes are shown below.

With the MPLS and frame-relay network up this is the route to the destination that is installed in the routing table.

Routing entry for 10.136.239.0/24

Known via "ospf 136", distance 110, metric 196, type intra area

Last update from 10.136.247.90 on Serial2/0.938, 00:01:30 ago

Routing Descriptor Blocks:

* 10.136.247.90, from 10.140.251.29, 00:01:30 ago, via Serial2/0.938

Route metric is 196, traffic share count is 1

With the frame-relay network shut down and just the MPLS network up this is the route to the destination that is installed

in the routing table.

Routing entry for 10.136.239.0/24

Known via "ospf 136", distance 90, metric 1

Tag 65000, type extern 2, forward metric 1

Last update from 10.136.15.15 on FastEthernet0/0, 00:02:51 ago

Routing Descriptor Blocks:

* 10.136.15.15, from 10.136.252.15, 00:02:51 ago, via FastEthernet0/0

Route metric is 1, traffic share count is 1

Does anyone know why the intra area route is prefered to the type extern 2 route eventhough the administrative distance is configured

to be less for the type extern 2?

Links to any documents about OSPF route selection would be a bug help?

Cheers

[a.awan]

OSPF will always prefer an intra area route to an external route. Administrative distances are used when comparing routes between different routing protocols or even when comparing routes learned from two different routing processes on the same router. As per RFC 2328 the order of route preference for OSPF is:

intra-area routes, O

interarea routes, O IA

external routes type 1, O E1

external routes type 2, O E2

So in your particular case the router is behaving like it should as per the OSPF standard. The following link is for the RFC itself:

http://www.cse.ohio-state.edu/cgi-bin/rfc/rfc2328.html#sec-11

Go to section 11 under the heading 'path-type'.

[dale.cooke]

that's great thanks for your help we will have to come up with another solution.

[Richard Burts]

The explanation by Atif was exactly right and to the point. As OSPF calculates from its link state data base and determines what routes to put into the routing table it has its order of preference and administrative distance does not come into play until the route is being considered for inclusion into the routing table.

One possible suggestion (which may work or may not work depending on the topology of your network) is to make the Frame Relay routes and the MPLS routes both appear to be external. You can do this by running two OSPF processes on your Frame Relay routers. One OSPF process includes the Frame Relay interfaces and learns the Frame Relay routes. The other OSPF process includes the interfaces connecting to your other routers. Redistribute the OSPF Frame Relay routes from one process into the other process which will advertise them to the other routers as external.

In fact if you do the redistribution of MPLS routes as OSPF type E1 and the redistribution of Frame Relay routes as type E2 the OSPF selection algorithm will prefer the MPLS routes if they are available without having to worry about manipulating metrics or anything else.

HTH

Rick