6.14 - How can I do equal-cost multipath routing?
Equal-cost multipath routing refers to having multiple routes in the routing table for the same network, such as the default route, 0.0.0.0/0. When the kernel is doing a route lookup to determine where to send packets destined to that network, it can choose from any of the equal-cost routes. In most scenarios, multipath routing is used to provide redundant uplink connections, e.g., redundant connections to the Internet.
The route(8) command is used to add/change/delete routes in the routing table.
The -mpath argument is used when adding multipath routes.

# route add -mpath default 10.130.128.1
# route add -mpath default 10.132.0.1
Verify the routes:
# netstat -rnf inet grep default
default 10.130.128.1 UGS 2 134 - fxp1
default 10.132.0.1 UGS 0 172 - fxp2

In this example we can see that one default route points to 10.130.128.1 which is accessible via the fxp1 interface, and the other points to 10.132.0.1 which is accessible via fxp2.
Since the mygate(5) file does not yet support multipath default routes, the above commands should be added to the bottom of the hostname.if(5) files for the fxp1 and fxp2 interfaces.
The /etc/mygate file should then be deleted.

/etc/hostname.fxp1
!route add -mpath default 10.130.128.1
/etc/hostname.fxp2
!route add -mpath default 10.132.0.1

Lastly, don't forget to activate the use of multipath routes by enabling the proper sysctl(3) variable.

# sysctl net.inet.ip.multipath=1
# sysctl net.inet6.ip6.multipath=1
Be sure to edit sysctl.conf(5) to make the changes permanent.
Now try a traceroute to different destinations. The kernel will load balance the traffic over each multipath route.

# traceroute -n 154.11.0.4
traceroute to 154.11.0.4 (154.11.0.4), 64 hops max, 60 byte packets
1 10.130.128.1 19.337 ms 18.194 ms 18.849 ms
2 154.11.95.170 17.642 ms 18.176 ms 17.731 ms
3 154.11.5.33 110.486 ms 19.478 ms 100.949 ms
4 154.11.0.4 32.772 ms 33.534 ms 32.835 ms

# traceroute -n 154.11.0.5
traceroute to 154.11.0.5 (154.11.0.5), 64 hops max, 60 byte packets
1 10.132.0.1 14.175 ms 14.503 ms 14.58 ms
2 154.11.95.38 13.664 ms 13.962 ms 13.445 ms
3 208.38.16.151 13.964 ms 13.347 ms 13.788 ms
4 154.11.0.5 30.177 ms 30.95 ms 30.593 ms

For more information about how the route is chosen, please refer to RFC2992, "Analysis of an Equal-Cost Multi-Path Algorithm".
It's worth noting that if an interface used by a multipath route goes down (i.e., loses carrier), the kernel will still try to forward packets using the route that points to that interface. This traffic will of course be blackholed and end up going nowhere. It's highly recommended to use ifstated(8) to check for unavailable interfaces and adjust the routing table accordingly.



source : http://www.openbsd.org/faq/faq6.html#Multipath