ECMP/multipath support

Ondrej Zajicek santiago at
Fri Oct 1 14:18:49 CEST 2010

On Fri, Oct 01, 2010 at 10:40:58AM +0100, Andrew Lemin wrote:
>    Hi Vincent,
>    This is something that we have also been after for some time too, and I
>    know that Securepoint (a company who implement BIRD in their
>    firewall/router product have also approached the bird team about), however
>    there does not seem to be any resources available in the BIRD team at the
>    moment willing to look into this despite how important it is.

Is ECMP really much important? It seems to me that it is useful
only in limited number of cases (like having two parallel lines
with similar capacities in a network). I would expect that
redundancy is usually solved by having generally denser network
graph (where ECMP is more likey a random coincidence). But it
seems that it is also a feature that is most often asked for.

I see two possibilities how to implement ECMP in BIRD:

1) adding special route type for a multipath route with multiple
gw addresses. This wouldn't be hard, it is consistent with a way
how Linux kernel handles multipath and works well with OSPF, but
it will not allow to 'merge' routes from different protocols
(a multipath route would have to be originated by one protocol).

2) allow BIRD kernel protocol to scan all equal-best routes to
one destination and 'merge' these to a multipath route for kernel.
This would be harder to implement, esp. because BIRD would not
allow more routes to the same destination from one protocol in
one routing table. But it is probably more natural way for a user.

What protocols are you thinking about with regard to ECMP?
ECMP in OSPF would work OK, ECMP in BGP probably would not work either,
i am not sure about RIP.

Elen sila lumenn' omentielvo

Ondrej 'SanTiago' Zajicek (email: santiago at
OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3,
"To err is human -- to blame it on a computer is even more so."
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