OSPF best route selection with same cost

[Ondrej Zajicek]

On Mon, Feb 17, 2020 at 02:36:52PM +0100, Patrik Rehuš wrote:
> Hello,
> I have a topology with multiple parallel path to same destination. It also
> use same next-hop router and each of them use default cost (not specified
> in config). In "show route" I can see there is only one active route so it
> probably means that BIRD don't use balancing between these routes. I cannot
> see in documentation exact algorithm used for route selection. Can you
> please explain me how is the best route selected when multiple paths with
> same cost are available? Is name of interface or IP address next criterion
> for selection? My goal is not balance traffic but I only want to
> understand how path selection works on equal links.

Hello

There is 'ecmp' option to enable equal cost multipath for OSPF. It is
enabled by default in BIRD 2.

When it is disabled, i think it is just first found path with that cost
during OSPF route calculation, which is based on link order in LSAs and
order of processing (distance) of intermediate routers. Link order is
based on order of interfaces in internal lists, which is based on order
these interfaces were detected/added.

-- 
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."