Deploying Cisco Express Forwarding – CEF-Based Multilayer Switching
What is multilayer switching?

Multilayer switching includes the ability to switch data based on information at multiple layers. Multilayer switching also refers to a class of high-performance routers that provide Layer 3 services and simultaneously forward packets at wire-speed through switching hardware. A Layer 3 switch performs packet switching, route processing and intelligent network services.

Layer 3 switch processing forwards packets at wire-speed by using ASIC hardware instead of microprocessor-based engines as might be found on a traditional router. Specific Layer 3 components such as routing tables or ACLs are cached into hardware. The Layer 3 packet headers of data traffic will be analyzed and packets forwarded at line speeds based upon that cached information.

Layer 3 switching can occur at two different locations on the switch:

  • Centralized switching – Switching decisions are made on the route processor by a central forwarding table, typically controlled by an ASIC.
  • Distributed switching – Switching decisions can be made on a port or line card level rather than on a central route processor. Cached tables are distributed and synchronized to various hardware components so processing can be distributed throughout the switch chassis.

Layer 3 switching takes place using one of these methods, which are platform dependent:

  • Route caching – Also known as flow-based or demand-based switching, a Layer 3 route cache is built in hardware as the switch sees traffic flows into the switch.
  • Topology-based switching – Information from the routing table is used to populate the route cache regardless of traffic flow. The populated route cache is called the Forwarding Information Base. Cisco Express Forwarding is the facility that builds the FIB

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