To determine the best path is the primary function of routing protocols, and this can be a
CPU-intensive process. Thus, there is a significant performance increase with the offload
of a portion of this function to switching hardware. This performance increase is the goal
of the MLS feature.
Two of the three major components of MLS are the MLS route processor (MLS-RP) and
the MLS switching engine (MLS-SE). The MLS-RP is the MLS-enabled router, which
performs the traditional function of routing between subnets/VLANs. The MLS-SE is a
MLS-enabled switch, which normally requires a router to route between subnets/VLANs.
However, with special hardware and software, MLS-SE can handle the rewrite of the
packet. When a packet transverses a routed interface, the change (rewrite) of non-data
portions of the packet occurs as the packet heads to the destination, hop by hop.
Confusion can arise here because a Layer 2 device appears to take on a Layer 3 task.
Actually, the switch only rewrites Layer 3 information and "switches" between
subnets/VLANs. The router is still responsible for standards-based route calculations and
best-path determination. You can avoid much of this confusion if you mentally keep the
routing and switching functions separate, especially when they are within the same
chassis (as with an internal MLS-RP). Think of MLS as a much more advanced form of
route cache, with a separation of the cache from the router on a switch. MLS requires
both the MLS-RP and the MLS-SE, along with respective hardware and software