10.3 Cisco Switching Mechanisms

CCNP RS 300-101, Implementing Path Control, Uncategorized

There are three methods for switching packets as they are accepted and forwarded out the router:

  1. Process Switching: The slowest and oldest method, where every packet forwarding decision is made by the CPU, or route processor. Is very resource intensive compared to the other two types.
  2. Fast Switching: Also called route caching, this is a case where the first packet in every traffic flow is inspected by the CPU and every subsequent packet is switched in hardware at the ASIC level. (Per source and destination IP pair.) Routing decisions are essentially cached in hardware after that first packet.
  3. CEF (Cisco Express Forwarding): Completely switched in hardware, this is the fastest switching method and is enabled by default in modern Cisco routers and MLS. A hardware forwarding table comprised of a Forwarding Information Base (FIB) and Adjacency Table are dynamically created at the data plane before any packets are subject to forwarding decisions, thereby expediting the process.

Note: not every packet is able to be forwarded by CEF, and in those cases, the packets are sent to the route processor. An example is CEF polarization in a topology using load-balanced L3 paths.

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