BGP Default Route into OSPF

Unanswered Question
Jul 28th, 2008

Hello,

I'm looking for the best solution for the following scenario:

CORE router is running OSPF along with R1 and R2.

R1 is border router and has working BGP session (full routes) and is directly connected to CORE router.

R2 is secondary border router, also has working BGP session (full routes) and is directly connected to CORE router.

R1 and R2 have no direct connection, use different ISP and ASs, both are connected to CORE and there's no iBGP running.

Now, I'd like to inject default route from the BGP into the OSPF, so CORE is aware of any changes on external links (R1 or R2) and uses the one that has working Internet connection.

What would be the next step? Shall I ask both ISPs to send me default routes via eBGP? Also, pointers on OSPF configuration would be appreciated.

Thanks,

Marko

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sundar.palaniappan Mon, 07/28/2008 - 16:18

Marko

Any recommendation would depend on how you would like to route traffic from the core out to the Internet. Do you want to load balance the traffic out both ISPs or use one as primary and the other one as backup connection?

Since both ISPs are advertising the full routing table redistributing all BGP routes into OSPF would help the CORE device choose the optimal path to get to any network on the Internet. However, the downside would be your CORE router would need lot of memory and processing power to make use of this option.

On the other hand if you want to use the ISP links as primary and backup then just configure the two border routers to advertise just the default route to your CORE. You can tweak the metric (cost) in OSPF to influence the route selection from the CORE perspective. You might be able to use a nice OSPF feature 'conditional default origination' in the backup border router to only inject a default route into the CORE only when the tracked WAN IP of the primary border router becomes unreachable.

Here's a link that you may find useful.

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00800945bf.shtml

HTH

Sundar

markopstc Mon, 07/28/2008 - 16:33

Sundar,

Thanks for the reply. Regarding your first question: there will be load balancing in place for utilization of both connections/links. Redundancy and failover if one fails is also a must.

Another thing that might be a issue is how CORE router will know which egress route to use when some of the eBGP connection fails. I can do PBR and next-hop config, but there's no redundancy when this happens.

I'm thinking of maybe using iBGP, but it requires full mesh (is it a must?) and I have no direct connection between R1 and R2. Any suggestions?

Regards,

Marko

sundar.palaniappan Mon, 07/28/2008 - 17:04

Marko

Expanding on this example of Load Sharing When Multihomed to Two ISPs Through Multiple Local Routers

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00800945bf.shtml#conf5

Yes you can run IBGP on all three routers to create a full mesh. No need to worry about the peers having to be directly connected as multihop peering is supported in IBGP by default.

However, this example only gives you a baseline understanding of how load sharing can be done in a situation like yours and the specifics or configuration required have to be worked out carefully as you don't want your AS to become a transit AS or result in suboptimal routing etc.

HTH

Sundar

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