Understanding Linux Network Internals

Concepts Behind Multipath Routing

Multipath is a feature that allows an administrator to specify multiple next hops for a given route's destination. In environments with substantial requirements, there are several reasons for doing this. A router could just use one ISP most of the time, and switch to the other when the first one fails for some reason. Another application of multipath is to keep a path on standby and enable it only when bandwidth requirements surpass a predefined threshold.

Figure 31-3 shows a topology where the network on the left is connected to the Internet via router RT, which is configured to use two uplinks simultaneously via two different ISPs.

Figure 31-3. Example of topology with multipath

Let's suppose we want to have RT use both RT1 and RT2 as default gateways, keeping them always available. On RT, we could define a multipath route, simply by providing the route with more than one next hop. The following user-space command, using the newer IPROUTE2 package, would enable multipath:

ip route add default scope global nexthop via 100.100.100.1 weight 1 nexthop via 
200.200.200.1 weight 2

Note that even if the route includes multiple next hops, the route is still considered a single route. Therefore, given a route (in our example, the default route 0.0.0.0/0) with more than one next hop, the kernel needs a mechanism to select the next hop to use each time the route ...