We originally designed our WAN to utilize IP CEF to load balance a pair of parallel T1s. Shortly after we needed to implement QoS on each of these two dual T1 routers. We were concerned that CEF would essentially negate our usage of Qos. So we decided instead to use MPPP to emulate a physical interface and make this relatively transparent to QoS.
We use QoS for data packets created by a Nortel 9150 VoIP gateway. This device encapsulates the voice into data packets before it reaches the ethernet so the routers will only know to accelerate delivery of these packets via QoS. We were concerned that CEF will deliver these packets out of sequence and such. Using a MPPP interface gives up buffering and reassembly of these packets at the interface before it is released to the router buffers.
Or maybe I'm off my rocker...
