bgp add paths to select only equal routes

[Alexander Zubkov]

Hello,

We aggregate routes learnt from bgp peers into multipath routes (ECMP)
and need to pass some of them to other peer. And there are
complexities now because we export not only the components of the best
multipath routes, but all others too, with lower local preference, for
example. If that peer is EBGP, the local preference is usually lost
and those routes become equal. I think we can now overcome this
limitation for our setup with some additional pain. But I think
feature like that would eventually be useful somewhere. Though, I am
not sure there is high demand for it from others. :)

On Thu, May 16, 2019 at 6:08 PM Ondrej Zajicek <santiago at crfreenet.org> wrote:
>
> On Wed, May 15, 2019 at 07:51:55PM +0200, Alexander Zubkov wrote:
> > Hello,
> >
> > If "add paths" config option is enabled for a bgp protocol than it
> > sends every route matching the filter on export. But I would like to
> > have an option to send only the best route with routes similar to it
> > (equal localpref, aspath length, ...). Like it is done in the kernel
> > protocol and its "merge paths" option. So for example if I had 2
> > routes for the same prefix with different bgp_local_pref or bgp_path
> > length, than only the route with better attributes would be announced
> > to the peer. Currently both routes would be announced.
>
> Hello
>
> That is true. For BGP ADD_PATH, all routes that pass filters are
> exported. We prefer to not add more export modes (like one for "merge
> paths") as that significantly complicates a part of code that is already
> too complicated. We plan to do some redesign there that should make it
> much simpler, so perhaps after that.
>
> Also, how such mode should behave and how much should be configurable?
> For example, "merge paths" applies to routes that are the same up to
> (including) IGP metric, on the contrary a basic use case for BGP ADD_PATH
> is to exchange feasible routes to/from route reflector and avoid issue
> with inconsistency and route hiding due to IGP metric being different on
> different speakers. For this case it would make sense to export routes
> with inferior IGP metric (and less important attributes), but not ones
> with inferior bgp_local_pref or bgp_path length.
>
> --
> Elen sila lumenn' omentielvo
>
> Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
> OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3, wwwkeys.pgp.net)
> "To err is human -- to blame it on a computer is even more so."