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

"Jerome Durand (jerduran)" <> Wed, 25 March 2015 20:32 UTC

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From: "Jerome Durand (jerduran)" <>
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Thread-Topic: [Idr] decraene-idr-next-hop-capability <-> ietf-idr-bgp-nh-cost
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Subject: Re: [Idr] decraene-idr-next-hop-capability <-> ietf-idr-bgp-nh-cost
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Hi Bruno,

Interesting there are some similarities to what I proposed here:
Where I tried to describe how we could carry next-hop capability to implement a host liveliness mechanism and solve the route-server black-holing stuff we have been talking about lately. Having a kind of general framework for any capability (ie. not just host liveliness) makes sense.

But your proposal relies on a non transitive attribute and couldn’t pass the route-server so I cannot adjust my proposal to fit in yours.

Anyway note that now I more or less stopped that work for the moment to focus on a more automated solution as there were some challenges with BGP (issues described in the draft). Also it seems that it was quite heavy from a processing/memory point of view to have new attributes per route. Note we had the idea to use some kind of "magic route" in order to have the attribute transported only once. Maybe to be investigated further. This is also described in the draft.



Le 25 mars 2015 à 20:56, <> <> a écrit :

> Hi David,
> Thanks for your comments.
> Please see inline.
>> From: David Lamparter [] > 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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