bgp add paths to select only equal routes

[Ondrej Zajicek]

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)
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"To err is human -- to blame it on a computer is even more so."