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

[Bruce Pinsky]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

David J. Hughes wrote:
> 
> 
> On 25/10/2005, at 9:29 PM, Kristofer Sigurdsson wrote:
> 
>>Upon further investigation, it seems that the BGP session with the
>>router that went down took it's time to time out, which meant the
>>former best routes stayed for some time.
> 
> 
> Yup, we had a thread on this recently.  By default an IOS based box 
> will require between 3 and 4 minutes to detect that the other "not 
> directly attached" router has gone away (i.e. no interface state 
> change).  This is the result of IOS using 60 second keepalives for the 
> BGP session and you need to miss 3 keepalives before the hold timer 
> expires.  Then factor in time for other paths to be advertised and 
> selected as best and you can be down for quite some time.
> 
> We are running trials with reduced keepalive times and things are 
> looking good.  I'll post a summary here once I have more confidence in 
> the results.  This is something you don't want to just randomly mess 
> with.
> 

An alternative to session timer tweaking is running a version that supports
BGP Next Hop Tracking and having your next hops propogated in your IGP.
Then a box failure or intermediate path failure that results in your next
hop becoming unreachable will cause the removal of routes that use that
next hop in just over the time it takes your IGP to converge and result in
the triggering of best path selection for those routes.

http://www.cisco.com/en/US/products/ps6350/products_configuration_guide_chapter09186a008045561e.html

- --
=========
bep

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.0 (MingW32)

iD8DBQFDXrxEE1XcgMgrtyYRArXCAKDhdFt6fzXT278SAyzUSf6oAkSHTwCg11H7
RjTr2HmHGeR2wm7/QSEIpI8=
=mt3c
-----END PGP SIGNATURE-----