BPG Decision Process and AD

Unanswered Question
May 28th, 2008

How does the Administrative Distance come into play with the BGP decision process.

a EBGP route has a AD of 20 while and IBGP route has a AD of 200.

The bgp decision process goes

Next Hop available,

Weight

LP

AS Path

Lowest Origin

Lowest MED

Prefer EBGP over IGP

etc...

So is step 7 and the AD the same? In other words, if you have two same routes, one EBGP and one IBGP, and they both have the same Weight, LP, AS Path, Origin, MED, then the administrative distance comes into play or step 7 in the decision? In other words, when in step 7, they talk about "prefer EBGP over IBGP, they mean pick the lower AD route?

I have this problem too.
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Richard Burts Wed, 05/28/2008 - 13:07

Timothy

Step 7 is really not talking about AD. In fact AD does not come into the BGP decision process at all. There is not any part of the BGP decision process that considers AD.

AD is used when a route is placed into the IP routing table but AD is not part of any routing protocol decision choice about which path to prefer within the routing protocol.

HTH

Rick

tsmarcyes Wed, 05/28/2008 - 14:16

So, let me get this straight. I'm assuming the AD for IBGP and EBGP is only used when comparing to other routing protocols? So for instance, you have a rip route and a IBGP route, thats when the AD comes into play. But when it comes to comparing IBGP to EBGP, then the AD has no relevance, and which route to choose is based on the BGP decision process.

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