[c-nsp] [Query] Bgp Session timers

[Jeff Tantsura]

Hi,

Not only that, don't forget BGP walker :)
The BGP scanner process normally runs every 60 sec

Default BGP Scanner Behavior

BGP monitors the next hop of installed routes to verify next-hop
reachability and to select, install, and validate the BGP best path. By
default, the BGP scanner is used to poll the RIB for this information every
60 seconds. During the 60 second time period between scan cycles, Interior
Gateway Protocol (IGP) instability or other network failures can cause black
holes and routing loops to temporarily form.

So if you don't use some kind of event driven notification like:
BGP Support for Next-Hop Address Tracking and BGP Support for Fast Peering
Session Deactivation there's a possibility that your traffic would be 
blackholed for up to 60 sec

Regards,
Jeff

> -----Original Message-----
> From: cisco-nsp-bounces at puck.nether.net [mailto:cisco-nsp-
> bounces at puck.nether.net] On Behalf Of Christoph Loibl
> Sent: vrijdag 1 juni 2007 8:53
> To: Simon Leinen
> Cc: Danny; cisco-nsp at puck.nether.net
> Subject: Re: [c-nsp] [Query] Bgp Session timers
> 
> Hi!
> 
> On Jun 1, 2007, at 12:21 AM, Simon Leinen wrote:
> 
> > Arie Vayner \(avayner\) writes:
> >> Danny,
> >> With iBGP the timers for BGP are not really important... You
> >> actually need to worry about the IGP convergence.
> >> The reason for that is that usually when a link fails, you don't
> >> really expect the BGP session to the RR to go down, but just use the
> >> redundant IGP path.
> >
> > Yes, but what about when a router fails, in particular a border
> > (eBGP+iBGP) router?
> >
> > In such a case, iBGP timers (or the configuration of a mechanism such
> > as BFD) will determine how long it takes for other routers that the
> > eBGP routes from the dead router have to be dropped.  This can be very
> > important, because using the dead router's eBGP routes can mean
> > blackholing traffic.
> 
> No need to adjust the iBGP timers in that case. Your bgp-next-hop
> (which is ideally a loopback-ip of the crashed router) will simply
> disapear from your internal routing-table, and all iBGP neighbors
> (also the RRs) will remove all routes that are unreachable (because
> of the unreachable next-hop) from their tables. You only rely on the
> convergence-time of your igp (ospf? isis? ...?) to remove the "dead"
> loopback-ip from its table and the calculation of BGP that is
> triggered when the loopback-ip disappears.
> 
> Stoffi
> 
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> 
> 
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