Administrative Distance and BGP Path Selection Process

Question:

I was looking at the BGP path selection WS-C3750V2-48TS-S Price 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.

Answer:

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.)"

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         WS-C3750V2-24TS-S