[c-nsp] BGP routes co-existing with different local-preference

[David Barak]

--- Alex Rubenstein <alex at nac.net> wrote:

> It may be nicer, but it's not better.
> 
> Doing one BGP session for multiple physical paths
> has an inherent danger. 
> Obviously, you have to staticly route the /32 of the
> far end BGP peer down 
> both paths, and there is always a chance that the
> physical path may be 
> dead, but show up/up on your router -- if this
> occurs, you will blackhole 
> approximately 1/n your traffic, were n is the total
> number of links you 
> have.
> 
> Running BGP between /30's or /31's over each
> physical path may not be 
> 'pretty' or 'sexy', but it is better.

I've got to disagree with you there: if you have 2 T1s
(simplicity), and statically route a /32 loopback down
each T1 (preferably to the interface, not to the
next-hop address), when one of the T1s goes down, the
/32 route is yanked from the FIB, and all traffic goes
down the remaining T1.  

I know several very large ISPs who follow this model,
and it works exactly as I'm describing it.

it's all about ebgp-multihop 2...



David Barak
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