Routing – Prioritize redistributed BGP routes over OSPF intra-area

bgpospfrouting

I have a few routers, each controlling a /16, connected together with VPN tunnels. These routers are all in the same OSPF area. Now, I'm also doing BGP with some other routers that I don't control, such that they may advertise routes to the other /16s, that I already get from the VPN tunnels.

Most of the time, the BGP routes are better than the OSPF ones, so I'd like far away routers to know about them. So I thought I'd redistribute the BGP routes into OSPF.

However, I then end up with externals in the OSPF database, which are never chosen over the intra-area OSPF routes.

Is there a way to have this work? Other than doing BGP between all the routers?

NOTE to anyone who replies: I can copy and paste Cisco documentation, too. Please consider writing an useful answer. Thanks.

Best Answer

Regardless of a route’s metric or administrative distance, OSPF will choose routes in the following order:

Intra-Area (O)
Inter-Area (O IA)
External Type 1 (E1)
External Type 2 (E2)
NSSA Type 1 (N1)
NSSA Type 2 (N2)

You simply cannot get OSPF to prefer an external route over an internal route.

Related Solutions

Split OSPF Area 0 Behavior – What happens

After more thoroughly combing through the RFC, I'm certain that the expected behavior I mentioned in my post is accurate.

Intra-area routing within each area (area 1, area 0(a), and area 0(b)) will work as expected.

Area 0 summaries will still flood to area 1 from each area 0 partition, giving area 1 all necessary routing information for both area 0 partitions.

Area 1 summaries will still flood to both area 0 partitions from area 1, allowing both area 0 partitions to know about area 1 routes.

Area 1 will not send area 0 summaries back into area 0, so split area 0 partitions will no longer know about each other and will not try to transit area 1 to get to each other (unless a virtual link through area 1 is created).

This is my desired behavior for a failure scenario based on my circumstances. If all of the redundant links between my data centers go down, or I need to perform certain types of maintenance, I don't want data center to data center traffic (mostly backups / off-site replication) saturating my branch site WAN links. I could apply policing on the WAN links for transit traffic between data centers, but the little bit of bandwidth I could give that traffic would be useless anyway - and that would require my team to maintain more config.

If anyone has any questions about this, feel free to comment and I will try to find the answers.

Cisco – OSPF Route Tagging and BGP AS Prepend

Based on your comment:

I'm trying to prevent any routes with a tag of 12000 from being redistributed into the MPLS.

Any routes redistributed into OSPF from EIGRP will be OSPF external routes. It is easy to redistribute only internal routes from OSPF into BGP:

router bgp 100
 redistribute ospf 1
!

Alternatively, you can use a route map to match a tag and deny redistribution:

route-map FROM_OSPF deny 10
 match tag 12000
route-map FROM_OSPF permit 20
!
router bgp 100
 redistribute ospf 1 route-map FROM_OSPF
!

You can also use a route map to set the routes with the tag to the BGP community no-advertise and/or no-export:

route-map FROM_OSPF permit 10
 match tag 12000
 set community no-advertise  no-export
route-map FROM_OSPF permit 20
!
router bgp 100
 redistribute ospf 1 route-map FROM_OSPF
!

Route-Maps for IP Routing Protocol Redistribution Configuration:

Best Answer

Related Solutions

Split OSPF Area 0 Behavior – What happens

Cisco – OSPF Route Tagging and BGP AS Prepend

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