Routing

How would I configure an OSPF routing preference based on the interface the routes are being advertised from?

Hi,

Can you please explain more details about your query?

I guess to be more detailed I have redundant connection to the internet. Each of these I am exporting my OSPF routes to. Since all routes are equal across OSPF, they are advertised equally through BGP regardless of the location should be prioritized as the best path.

So if I have an interconnected network using OSPF to propagate my internal routes. I then take those internal routes and want to send all internal routes to all DIA circuits for redundancy. If I have a location in LA another in Denver and another in Dallas I do not want traffic destined to a location in Dallas to prefer to come on the Denver DIA.

I was thinking this should be done from the internal OSPF routes, but after reconsidering I think it have to do with both OSPF preferences internally and also BGP advertisements.

Hi,

I am not sure if I understood your requirement completely, but you said that you are exporting OSPF routes to internet peers using BGP. So I guess you must be using an export policy to redistribute OSPF routes into BGP? If that is the case, you can use metric or as-path-prepend in BGP to achieve the required preference changes.

You can refer to following links for more details:

Configuring a Routing Policy to Prepend the AS Path

Configuring BGP MED

Thanks

I guess a picture may help. In the attached drawing everything is peered via BGP from each of the core networks. The blue areas are all connected via OSPF. The OSPF routes are being export into the BGP and the advertised to the Internet. Currently if the connection between Network Core 1 and the Internet goes down traffic is rerouted through the Network Core 2 router and the Internet since the routes from the attached networks of Network Core 1 are also being advertised there. If the connection to Network Core 1 then comes back on line the traffic will continue to travel through Network Core 2 because the weight for the Network Core 1 route has the same weight whether they are going the Core1 or Core2. I need the routes from the attached networks to go through Network Core 1 when that connection comes back up.

How can I do a BGP AS prepend based on the source of the OSPF route?

Attachment(s)

Routing Diagram.pdf   194 KB 1 version

try this
set policy-options policy-statement ospf2bgp term mt from protocol ospf
set policy-options policy-statement ospf2bgp term mt from interface ge-0/0/5.0
set policy-options policy-statement ospf2bgp term mt then preference 20
Instead of using the interface you could also use route filter statement

The only modification I made was to do an as-path-prepend instead of metric since the prepend is propogated throughout the internet.