[c-nsp] IOS XR / advertise best-external

[Adam Vitkovsky]

> Dan Peachey
> Sent: 18 May 2015 22:04
> 
> On 18 May 2015 9:40 pm, "Gert Doering" <gert at greenie.muc.de> wrote:
> >
> > Hi,
> >
> > On Mon, May 18, 2015 at 09:23:50PM +0100, Dan Peachey wrote:
> > > If that's the case I find it strange why that's now the default
> behaviour.
> > > Surely that's the point of 'additional-paths' to control what routes you
> > > want to have backups for. In my case I definitely wouldn't have wanted
> > > backup routes for everything by default.
> >
> > BGP add-path is for sending multiple routes for a single prefix towards a
> > BGP peer.  Different thing again...
> >
> > gert
> 
> You can use 'additional-paths selection' on its own without 'send' or
> 'receive' to filter what backup routes you install in the RIB/FIB. Using
> this on its own does not negotiate add-path with any peers or advertise
> multiple routes.
> 
> Unless I missed the point, the default behaviour of enabling
> 'best-external' now seems to be to install the backup route in the RIB/FIB,
> which was not the case last time I looked at it and may not be desired
> behaviour.
> 
> Regards,
> 
> Dan
> 

I'm pretty sure that even on 4.3.4 for eBGP prefixes you need to use the "additional-paths selection route-policy" to get a backup route installed.
Though I'm not that sure/can't recall what the behaviour was for iBGP prefixes, though flooding FIB with all the backup routes is certainly not desirable.

adam
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