In an ECMP configuration, because of load balancing, return traffic could enter through a different interface than the one it exited from. If this happens, the DP processor operating with default interface-based session tables may not be able to send the return traffic to the FPC that processed the incoming session, causing the return traffic to be dropped. Operating with VDOM-based session tables solves this problem, allowing traffic received on a different interface to be properly identified and sent to the correct FPC.
Enabling VDOM session tables can reduce connections per second (CPS) performance so it should only be enabled if needed to support ECMP. This performance reduction can be more noticeable if the FortiGate-6000 is processing many firewall only sessions. If the FortiGate-6000 is performing content inspection where CPS performance is less important, the performance reduction resulting from enabling VDOM-based session tables may be less noticeable.