Equal Cost Multi Path (ECMP)

The Linux networking stack supports Equal Cost Multi Path (ECMP) by adding multiple next-hops to the route.

ECMP Routes

To install a route with multiple next-hops:
ip route add 192.168.2.0/24 nexthop via 192.168.0.2 dev sw1p1 weight 1 nexthop via 192.168.1.1 dev sw1p1 weight 1

To edit an existing next-hop configuration, you need to delete the entire ECMP route and then re-add it with a modified next-hop configuration.

Linkdown Next-hops

When the netdevice goes down, the next-hops that are using it are marked with a linkdown flag. By default, the routing subsystem tries to forward packets through them. This behavior is defined by sysctl setting net.ipv4.conf.default.ignore_routes_with_linkdown.

To set the kernel routing subsystem to exclude next-hops with link down from its ECMP group, use the following setting:
$ sysctl -w net.ipv4.conf.sw1p1.ignore_routes_with_linkdown=1

Setting net.ipv4.conf.default.ignore_routes_with_linkdown=1 in the sysctl configuration files will make this option the default for all netdevices on the system.

Multi-path Hash Policy

Setting sysctl to net.ipv4.fib_multipath_hash_policy controls which hash policy to use for multipath routes.
This setting is ignored by the Switchdev driver. L4 (5 tuple) hash is always used.

See also

• About routing: Static-Route.md