Subject: Re: [Idr] decraene-idr-next-hop-capability <-> ietf-idr-bgp-nh-cost

From: "Jakob Heitz (jheitz)" <[email-masked]>
Date: Tue, 29 Sep 2015 20:25:49 +0000

Another benefit of advertising the next-hop capability in the same
update as the prefixes it applies to is that the receiving router
then will know the capabilities at the time that it receives the
prefixes.

If this information were to come in a different update message in
a different SAFI, it will not know how to treat the prefix until
the update containing the next-hop capability is received.
The next-hop SAFI update could come at any time before or after
the prefix update. BGP updates are not synchronized unless the
NLRI is the same, especially if route reflectors or confeds are used.

If there are multiple paths to a prefix and a new path with
unknown nexthop capabilities becomes bestpath, traffic loss can
occur until the nexthop capabilities are learnt.

Therefore, it is best to keep all information about how to treat
a prefix in the update message that advertises that prefix.

--Jakob

> -----Original Message-----
> From: Idr [mailto:[email-masked]] On Behalf Of [email-masked]
> Sent: Wednesday, March 25, 2015 12:57 PM
> To: David Lamparter <[email-masked]>
> Cc: idr wg <[email-masked]>; [email-masked]; [email-masked]
> Subject: Re: [Idr] decraene-idr-next-hop-capability <-> ietf-idr-bgp-nh-cost
> 
> Hi David,
> 
> Thanks for your comments.
> Please see inline.
> 
> > From: David Lamparter [mailto:[email-masked]] > Sent: Tuesday, March 24, 2015 4:30 PM
> >
> > Hi Bruno, Ilya, Robert,
> > Hi idr list,
> >
> >
> > the next-hop-capability draft currently seems to target using the new nexthop
> > capability on a per-advertisement level.  Considering that draft-idr-bgp-nh-
> > cost adds a new SAFI to convey information about nexthops, it seems natural
> > to tranposrt the capability information in that place?
> 
> That's a valid comment.
> 
> 1)  cost/benefit
> Avertising the next-hop-capability on a per update basis has indeed a cost but also a benefit.
> In term of cost, the simplest next-hop-capability is 5 octets. Compared to an update in the range of 100-1000 octets,
> that's 5% to 0,5%.
> In term of benefit, this allow customizing the capability on a per update/routes granularity if required, which may
> be a useful possibility for some capabilities.
> Engineering is about trade-off. IMO, the benefit, especially long term, out weight the fixe cost, which is small and
> will become smaller, in relative term, in the future (thanks to Moore's law).
> 
> 2) deployability
> In addition, in term of deployability, a new attribute has no impact.  On the other hand draft-idr-bgp-nh-cost use a
> new AFI/SAIF which requires reconfiguration and reboot of all BGP sessions. This is a much higher cost for IBGP
> alone. For EBGP, this is simply not an option IMO.
> 
> 
> So, I agree that this is a valid option to consider, however I don't think that it's a better one.
> 
> > (That would also remove the question of what behaviour should be when you
> > get different advertisements with the same nexthop, but with a different
> > nexthop capability attribute.)
> 
> For a given route/NLRI, you use the next-hop capability attached to this route.
> To rephrase, you do not consider next-hop capability that may have been advertised in another update.
> I agree that this may not be crystal clear in the text, I will clarify in the next revision.
> 
> Thanks again for your comments,
> Bruno
> 
> >
> >
> > -David
> 
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