The conventional routing logic described so far operates very much like any conventional router, with the addition of some handling for bonding and duplicate subnets.

However the FB2500 also allows the possibility of route overrides which control routing in more more detail. This feature is part of session tracking functionality, and so applies on a per-session basis (contrasting with the per-packet basis for the conventional routing). For details on sessions, and session-tracking, refer to Chapter 7.

When establishing a session it is possible to scan an ordered list of rules which can consider not only the target IP but also source IP, protocol, ports, and interfaces being used. The result is (typically) to set a routing target IP for the session (and possibly a routing table to jump between tables).

This is done for each direction on the session and remembered. This new target IP is then used on a per packet basis in the same way as above instead of the destination IP address of the packet. This is the same as set-gateway in the normal session tracking logic. However, routing overrides are applied at the end of checking rule-sets and applied both ways, allowing, in effect, a set-reverse-gateway.