More than one WAN router at the same site raises the possibility of asymmetric routing. Normally, IP networks are not affected by what path the packets take, so long as they arrive at their destination. However, the appliance relies on seeing every packet in the connection. "End-around" packets are not acceptable.
In a site with only one WAN router, asymmetric routing is not a problem, because the appliance can be placed in the path between the router and the rest of the site, so that traffic into or out of the router also passes through the appliance. But with two WAN routers, asymmetric routing can become an issue.
Asymmetric routing problems can appear during installation or later, as a result of failover to a secondary link, or other forms of dynamic routing and load balancing. The following figure shows an example sites that might suffer from asymmetric routing. If sites C and D always use the direct path, C-D or D-C, when sending traffic to each other, everything is fine. However, packets that take the longer path, C-E-D or D-E-C, bypass the appliances, causing new connections to be unaccelerated and existing connections to hang.
Asymmetric routing can be addressed by router configuration, appliance placement, or appliance configuration.
If the router is configured to ensure that all packets of a given connection always pass through the appliance in both directions, there is no asymmetry.
If the appliance is positioned after the point where all the WAN streams are combined, asymmetry is avoided, and all traffic is accelerated, as shown in the following figure.