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

<bruno.decraene@orange.com> Wed, 25 March 2015 19:56 UTC

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From: <bruno.decraene@orange.com>
To: David Lamparter <equinox@diac24.net>
Thread-Topic: decraene-idr-next-hop-capability <-> ietf-idr-bgp-nh-cost
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Date: Wed, 25 Mar 2015 19:56:48 +0000
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Subject: Re: [Idr] decraene-idr-next-hop-capability <-> ietf-idr-bgp-nh-cost
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Hi David,

Thanks for your comments.
Please see inline.

> From: David Lamparter [mailto:equinox@diac24.net] > 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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