From Scratch #3: Redistributing Between OSPF and BGP

This is a pretty common scenario. You are running and IGP (in this case, OSPF) on your local network and you connect to another company or ISP via an eBGP link.

Your goal is to leak some of the external routes received via BGP into your IGP, and in turn leak some of your internal routes out via BGP. The constraint is you don’t want everything. Only the routes you decide.

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BGP vs IGP

I’m not going to lie. The first time I used BGP it confused the crap out of me. If you come from an enterprise environment (like me) then you are probably pretty familiar with  how IGPs (for me it was OSPF) behave – if not how they work.

You insert a network statement and your IGP of choice starts working on all the interfaces within that network statement and hey presto – everything more or less straight up works. You don’t really need to understand how LSAs are propagated to get OSPF up and running.

Then one day you will undoubtedly have to configure BGP. BGP is a fairly different beast. The commands work differently, the protocol works differently.

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