[c-nsp] Choosing iBGP over eBGP

[Tim Franklin]

> You are conflating two steps of the decision process. External vs 
> internal as a selction criteria is relevant for origin codes only,
> when taking a walk through the BGP tabel to decide what gets put 
> into the routing table.

Nope.  External vs internal as an origin code is higher up the chain than
eBGP vs iBGP, but *both* are potentially considered in the decision.

http://www.cisco.com/warp/public/459/25.shtml, origin codes are step 5, BGP
"type" is step 7.

> > I've even tried to lower the local preference for the eBGP path 
> > without much success. 
> > Any tips out there that I can use to make this work? Thanks.
> 
> What you're describing sounds like you are running with default 
> administrative distance.  Refer to to:
> http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09
186a0080094195.shtml#topic2

I've seen this one come up a lot, and it seems to confuse a lot of people.
Admin distance is only relevent when comparing *between* routing protocols,
not within BGP.

If you've received a route for the same prefix via eBGP and iBGP, the admin
distances are not used by BGP at all when deciding which is the best BGP
route to use.  It's all done based on the decision steps at the earlier URL
(which does compare eBGP vs iBGP, but *not* via admin distance).

Once BGP has picked a best-BGP route, *then* the admin distance attached to
eBGP or iBGP is used to see whether that route is preferred (and hence
installed in the routing table) against the same route learnt from OSPF,
RIP, EIGRP etc.

We still need to see 'show ip bgp <prefix>' from the OP to work out why it's
making the decision it is.

Regards,
Tim.

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____________   Tim Franklin                 e: tim at colt.net 
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