When the FortiGate unit selects an ECMP route for a session, a route cache is created that matches the route with the destination IP address of the session. All new sessions to the same destination IP address use the same route until the route is flushed from the cache. Routes are flushed from the cache after a period of time when no new sessions to the destination IP address are received.

The route cache improves FortiGate unit routing performance by reducing how often the FortiGate unit looks up routes in the routing table.

If the FortiGate unit receives a large number of sessions with the same destination IP address, because all of these sessions will be processed by the same route, it may appear that sessions are not distributed according to the ECMP route failover and load balancing configuration.