Switching Tables (Part-3)

CAM and TCAM Tables

On Cisco switches, the MAC address table is stored in Content Addressable Memory (CAM).

CAM differs from the more prevalent Random Access Memory (RAM):

• RAM queries a specific memory address, and then returns the data or content stored at that address location.

• CAM operates essentially in the reverse, and does not require that a memory address be provided. Instead, CAM queries for the desired content, and then returns all matching results, including any associated content.

CAM is significantly faster than RAM, as it searches the entire memory content in one cycle, instead of a single address at a time. However, CAM is more expensive than RAM. When performing a MAC address table lookup, the MAC address itself is the content being queried. For any matching results, CAM will return the destination port (the associated content).

Cisco uses the terms MAC address table and CAM table interchangeably. This guide will use the term CAM table moving forward. Idle entries in the CAM are purged after 300 seconds, by default. This timer is reset every time a frame is received with the associated MAC address on the correct port.

If a host moves to a different port on a switch, the CAM table entry for the previous port will be purged immediately. This is desirable behavior - a MAC address is unique, and should never exist on more than one switch port unless a switching loop or other issue exists.

Ternary Content Addressable Memory (TCAM) tables provide highspeed lookups for two additional functions:

• Filtering traffic using access-lists

• Prioritizing traffic using QoS

TCAM tables are covered in greater detail later in this guide. Multilayer switches utilize the Forwarding Information Base (FIB) table for L3 forwarding decisions.