Administrative Distance and BGP Path Selection Process

[Venison Mogambi]

I was looking at the BGP path selection criteria (weight, local_pref, MED, etc.) and I noticed that the seventh attribute considered is whether the prefix was learned from eBGP or iBGP. But the Administrative Distance for eBGP is 20, versus 200 for iBGP, and doesn't AD trump everything in the route selection process? So shouldn't eBGP vs. iBGP be the very first attribute considered in the BGP path selection process? It seems like a contradiction to me.

Any feedback is appreciated. Thanks.

[Bilal Nawaz · Accepted solution]

Hello, I came across this question before (i questioned this too) but found Brian McGahans answer which is a good one in my opinion....

"Actually it doesn’t come into play at all. iBGP and EBGP distances are never compared for the same prefix. The distance only comes in when you’re comparing BGP to another protocol, like iBGP to EIGRP, or EBGP to OSPF. In the BGP Bestpath Selection you prefer External (EBGP) over Internal (iBGP) routes, but changing the distance doesn’t affect this. For example if you set the iBGP distance to 19 (lower than EBGP), the bestpath selection will still choose the EBGP route over the iBGP route, assuming that everything is equal up until that point (weight, local pref, as-path, origin, med, etc.)"

Hope this helps

Reference: http://ieoc.com/forums/t/22404.aspx

Sent from Cisco Technical Support iPhone App

[Venison Mogambi]

Hmm...that's interesting. Although I don't see how he can say "iBGP and EBGP distances are never compared for the same prefix". What if two iBGP peers (let's say Router A and Router B) were both peered to the same neighboring AS? Router A might learn about a remote prefix from its eBGP peer in the neighboring AS, but Router A also might learn about that same prefix via iBGP from Router B. In that case, wouldn't Router A be learning about the same remote prefix via both eBGP and iBGP?

[cadet alain · Accepted solution]

Hi,

AD only come into play when receiving same prefix from different routing processes and iBGP as well as eBGP are BGP.

In this case the BGP router will choose the best path to install in the RIB following the best path selection algorithm:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml

Regards.

Alain

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[Venison Mogambi]

Yeah, that makes sense. I guess it's confusing because both iBGP and eBGP have their own AD, so it seems like they might be considered against each other. Thanks for the clarification.