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REDISTRIBUTING BGP TO OSPF AND VICE VERSA

Posted on 2013-10-30
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Last Modified: 2016-06-03
We have two 7200 Internet routers connected to the internet running BGP.  We have two 3750, one connected to each 7200 router.  Currently we are using HSRP but would like to implement OSPF in the DMZ to allow redundancy.  We elected to not use GLBP...

We planned to redistribute BGP into OSPF, and OSPF into BGP.  However, we are aware the internet routing table can be as large as 25 megabytes in size... way too large for the 128 meg of the 3750 layer 3 switches.  Can you provide a sample configuration and explanation of how we can prevent the BGP routes from over subscribing the 3750.  I thin we need to use DEFAULT-INFO ORIGINATE  command within the OSPF routing protocol route process.  Please provide guidance.  Much thanks... Chas
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Question by:ITCharles
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pergr earned 500 total points
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Yes, if you have the full internet routing table, it is around 450,000 routes  - which is too much to have in OSPF on almost any router, and most certainly on the 3750.

As you mention, the best thing to do is to originate a default route in OSPF from the 7200 routers.

I assume the two BGP routers are directly connected to each other, and also run iBGP with each other. In that case you can have both originate a default route - and once any router receives a packet, the BGP routers will select the best router to use for exit.

The ospf command to use is "default-info orignate always", which will make the router advertise a default route in OSPF even if it does not have a default route in BGP (or any other protocol).

If you do have also a default route (0.0.0.0/0) in BGP, then you can use "default-info originate", and the default route in OSPF will be removed if the default route dissapears in BGP. However, I believe that is not necessary in your case.
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