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BGP routes when OSPF routes are unavailable


Here is my simplified network layout (AS and IPs are made up for privacy):

As you can see, we own one /20 and we have two locations connected to two different Internet peers. Network 1 is announcing the whole /20 and Network 2 is announcing the same IP space broken up in two /21. The more specific BGP announcement wins. Both locations are also connected via a Metro Ethernet connection and both sides are part of OSPF area 0. Both locations are also BGP peering with each other. Routing between both locations works fine, so does the BGP routing to our peers. Should Internet Peer 1 fail, traffic makes it just fine to Network 1 via Internet Peer 2 and it's connected Metro ethernet. So, basically, everything is working great as long as the Metro Ethernet link between Network 1 and Network 2 is up. If the Metro Ethernet connection fails, I can no longer reach resources on the other network. The plan is that traffic should go over the internet if the Metro Ethernet connection is down. When I do a traceroute (while the Network Ethernet is down) from Network 1 to Network 2, the route goes to Null 0. Since OSPF can't communicate with the other network traffic destined to the other network goes to Null0. However, BGP still has a valid "default route" to the internet. Also, please note that traffic originated from the internet and destined to other network, will reach the destination just fine.

The question is: What do I need to do so that I can still reach the other network from each location when the Metro Ethernet connection fails?

I can post configs if needed.

Thanks for your help



Problem solved.


I have followed Paul's advise and created a GRE tunnel between the two routers and enabled OSPF (with a higher path cost) over the GRE tunnel.

When the Metro Ethernet circuit is going down, routes will continue to be advertised between both routers via OSPF and will take priority over the BGP aggreate Null routes. 

This has been tested and works as intended.

Thank you for confirming that the GRE tunnel has been implemented, tested, and is working and does solve the problem. Thank you for marking this discussion as solved.