From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Wed Jun 28 2006 - 13:53:51 ART
Stephen,
Per RFC 4271 the propagation delay for an advertisement should
be based on how fast the device can process updates and run bestpath
selection (function of cpu, memory, etc) and the
"MinRouteAdvertisementIntervalTimer", which is similar to SPF delay in
OSPF:
<rfc>
9.2.1.1. Frequency of Route Advertisement
The parameter MinRouteAdvertisementIntervalTimer determines the
minimum amount of time that must elapse between an advertisement
and/or withdrawal of routes to a particular destination by a BGP
speaker to a peer. This rate limiting procedure applies on a per-
destination basis, although the value of
MinRouteAdvertisementIntervalTimer is set on a per BGP peer basis.
</rfc>
The notion that iBGP converges faster than EBGP is per the RFCs
recommendation that:
<rfc>
Since fast convergence is needed within an autonomous system, either
(a) the MinRouteAdvertisementIntervalTimer used for internal peers
SHOULD be shorter than the MinRouteAdvertisementIntervalTimer used
for external peers, or (b) the procedure describe in this section
SHOULD NOT apply to routes sent to internal peers.
</rfc>
The problem is that you get into the meaning of SHOULD and
SHOULD NOT as the RFC is just a recommendation, and the vendor doesn't
necessarily have to follow it.
In this particular case Cisco does implement this feature with
the configurable timer "neighbor advertisement-interval"
<CCO>
neighbor advertisement-interval
To set the minimum interval between the sending of Border Gateway
Protocol (BGP) routing updates, use the neighbor advertisement-interval
command in address family or router configuration mode. To remove an
entry, use the no form of this command.
Defaults
30 seconds for external peers and 5 seconds for internal peers.
</CCO>
The risk of changing this timer is that if you have rapid
UPDATE/WITHDRAW messages and you propagate them to your EBGP peers you
run the risk of having your prefixes dampened.
HTH,
Brian McGahan, CCIE #8593
bmcgahan@internetworkexpert.com
Internetwork Expert, Inc.
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> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> Skinner, Stephen
> Sent: Wednesday, June 28, 2006 11:01 AM
> To: 'ccielab@groupstudy.com'
> Subject: IBGP v EBGP , quicker convergence
>
> guys,
>
> I have tried the CCO and I have tried the archive and have had no luck
,
>
> This is not a nice question , so I apologise in advance.
>
>
> I have a BGP network and I want the fastest convergence (when a route
is
> missing) as possible. (WITHOUT modifying anything in BGP , just using
the
> defaults)
>
> can i first confirm a couple of things.
> in a single BGP router.
>
> imagine full mesh. 1 route map to allow in networks
>
> 1.If i add a network to a route map , and nothing else
> the BGP scanner will pick this up after a potential 60secs (depending
on
> the
> timing cycle)
> and then advertise it out to its peers (providing its allowed) yes ?
>
> i should not need to do a "clear ip BGP blah" command.
>
> 2.If a network goes missing , an update will be sent to BGP neighbours
to
> tell them about this.
> this will only happen when the BGP scanner picks this up , which in
theory
> could be 60 seconds yes ?
>
>
> the reason i ask is because of the following.
>
> i have been told that IBGP will converge faster than EBGP
>
> the reason for this is that a BGP command "update interval" which can
> modify how often updates are sent.
> and that by default an IBGP neighbour sends out an update every 5secs
> .
>
> but for EBGP it sends it out every 30sec.
>
> so that would mean IBGP converges quicker than EBGP .
>
> i cant find this command on the CCO .
>
> also i believe this is in not related to convergence after a route is
> lost.
> but is related to how often standard updates are sent.
>
> i believe EVEN if there is such a command , when i loose a route ,
BOTH
> IBGP
> and EBGP will send out a update (because triggered updates is on by
> default)
> at the same time,
> therefore both IBGP and BGP will converge at the same time.
>
> am i wrong? , to be honest i don't know .
>
>
> TIA
>
>
> Stephen Skinner
>
>
>
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This archive was generated by hypermail 2.1.4 : Sat Jul 01 2006 - 07:57:33 ART