[PATCH,RFC] Allow exchanging LOCAL_PREF with eBGP peers.

[Job Snijders]

Hi,

Can you make the LOCAL_PREF something that can be matched on? Example:

if ! bgp_path.local_pref = 80 then {
bgp_local_pref = 100;
}

This way one can limit the influence of the adjacent neighbor to either 80 or 100 (default).

Kind regards,

Job

On 18 Feb 2017, 17:07 +0100, Lennert Buytenhek <buytenh at wantstofly.org>, wrote:
> On Sat, Feb 18, 2017 at 12:27:22PM +0100, Tim Weippert wrote:
>
> > Hi Lennert,
>
> Hello!
>
>
> > > I've attached a patch that allows (selectively) exchanging LOCAL_PREF
> > > with eBGP peers.
> >
> > a perfect timing. Yesterday i thought about that ..
> >
> > > The BGP RFC (RFC4271) says that you shouldn't send LOCAL_PREF to eBGP
> > > peers, but most modern BGP implementations have an override to allow
> > > doing this anyway, and it is very useful in some scenarios, for example
> > > if you have a network topology a la RFC7938.
> >
> > [ ... ]
> >
> > > I'm not submitting this patch for inclusion yet at this point (which is
> > > why there is no documentation), but this kind of functionality is useful
> > > for us, and I'd like to hear from people whether they think this could
> > > be useful for them, too. And of course, there will have to be a lot of
> > > bikeshedding about the name of the config option! :-)
> >
> > I thought about it yesterday as it could be helpfull in my situation, i
> > then come to the solution to use MED and/or an Community Setting which
> > will trigger the LOCAL_PREF in the foreign AS (eBGP Peering).
> >
> > I think that is today a common way to do this. So i see the need but
> > there exists some ways which will be more standardized within BGP!?
> >
> > If i had all eBGP AS Peerings under my control, it may be usefull to do
> > it directly with a LOCAL_PREF spreaded over the ASN, if not i think you
> > want enable it, as the foreign AS can set it to any value it want/think.
> > And you had to filter it again to not clash it with internal AS usage of
> > LOCAL_PREF?!
> >
> > So for general implementations i think it isn't needed/usefull, but for
> > some special purposes it would be great to had the possibility to send
> > LOCAL_PREF towards eBGP borders.
>
> Thanks for your feedback!
>
> The idea here is that all eBGP peers are under control of one
> organisation, but you want to speak eBGP and not iBGP between them
> because eBGP gives you certain benefits over iBGP, such as a more
> sensible loop detection algorithm. RFC7938 documents some cases
> where you would want to do this and our use case is roughly similar
> to this, and it's a fairly common design pattern in datacenter
> networks.
>
> It's generally a bad idea to accept LOCAL_PREF from an eBGP peer
> that is not under your control, for reasons you mentioned and others.
> My patch doesn't accept LOCAL_PREF from an eBGP peer unless you
> specifically configure bird to do so (using 'ebgp localpref rx'), and
> if you don't enable that option, bird will do what it always did, which
> is to ignore the peer's value and fill in default_local_pref (which
> will be 100 if you didn't change it).
>
>
> Cheers,
> Lennert
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