Harold Arley Morales has asked an interesting question:

What's the difference between Cisco Express Forwarding and Cisco MLS? Is Cisco's implementation of MLS standardized?

CEF is a routing table lookup mechanism. Instead of doing a lookup in the main IP routing table (displayed with the show ip route), the router does a lookup in a fully computed non-recursive version of the IP routing table (Forwarding Information Base - FIB) with layer-2 next-hop information attached to it (adjacency table).

MLS is a caching mechanism (similar to Netflow) that offloads layer-3 processing from the routing component into layer-2 ASICs that cannot perform full-blown layer-3 switching. When the layer-2 engine detects a single IP packet traversing multiple VLANs, the MLS populates the cache with the flow details and the subsequent packets belonging to the same flow (same source/destination IP addresses and port numbers ...) are switched without going through all the layer-3 mechanisms (for example, access lists). The Multilayer Switching Overview document gives you additional details.

The MLS uses a proprietary protocol (MLSP) through which the layer-2 switches identify routers.

This article is part of You've asked for it series.

Update 2008-12-08: Ofer Granit sent me the following information: according to Troubleshooting IP Multilayer Switching document, Supervisor Engine 2 and Supervisor Engine 720 no longer use MLS but rely exclusively on CEF to perform layer-3 forwarding.