OSPF as iBGP

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Aug 18th, 2008
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If were to use OSPF as internal BGP, would we need to create the ajacencies between the int on the router or can we just passive-interface them?


Thanks..

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Edison Ortiz Mon, 08/18/2008 - 13:36
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I believe you were referring to OSPF as the IGP, not the internal BGP as that is another routing protocol altogether :)


If you don't want to create adjacencies between routers running OSPF while having OSPF enabled in the router, you can use passive-interface command under the OSPF routing process referring to the interfaces connected to the unwanted neighbors.


HTH,


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Edison.

axfalk Wed, 08/20/2008 - 06:11
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Edison, thnx for your response.

You mentioned "unwanted neighbors" in your reply - can you please elaborate? Also, what would be the trafe-off in having adjaciencies created with regard to using OSPF as the IGP for the eBGP?


Thanks again..

Edison Ortiz Wed, 08/20/2008 - 07:22
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There are times when you need to enable OSPF in a router and you want to make sure a certain interface does not send or receive OSPF hello packets. In that case, the passive-interface command will prevent that interface from establishing adjacency to/from unwanted neighbors.


As for the 2nd question, I really don't understand what you are asking.


For eBGP to work, it needs reachability to its neighbor. If its neighbor is directly connected, you don't need an IGP such as OSPF. If the neighbor is multiple hops away or you are peering with loopbacks, then you need an IGP to know how to get there.


HTH,


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Edison.


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axfalk Wed, 08/20/2008 - 07:52
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Thanks for your reply...We're peering with the interface addresses as opposed to the loopbacks as the neighbors are multiple hops away and are in different AS'es...We're using OSPF for the reachability to the neighbors and I was just wondering if we have to create ajacencies between them for OSPF to function properly..


Thanks again..

Edison Ortiz Wed, 08/20/2008 - 08:04
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I was just wondering if we have to create ajacencies between them for OSPF to function properly..


One protocol aids the other. In the case, OSPF provides the information for the BGP process to know about its neighbor.


Think of someone telling you their home is at 123 Main Avenue but w/o telling you how to get there.


123 Main Avenue on this example is the BGP neighbor statement - the 'how to get there' information is provided by OSPF.


However, if 123 Main Avenue was next to your home at 122 Main Avenue, you don't need to know how to get there (directly connected) - hence you don't need OSPF.


In your environment, you need OSPF so BGP can function properly, not the other way around.


HTH,


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Edison.

axfalk Wed, 08/20/2008 - 17:50
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Edison, thanks..So, if I understand your analogy correctly, we do need the ajacencies created?


thanks again..

Edison Ortiz Wed, 08/20/2008 - 17:51
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Yes.

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