TCAM on Catalyst switches

Catalyst switches have an interesting internal architecture that uses a Ternary Content Addressable Memory (TCAM) to perform a variety of lookups. For example:

  • If a CAM entry matches on destination MAC address, it performs L2 switching (aka bridging)
  • If it matches on destination IP address, it performs L3 switching (aka routing)
  • If it matches on a combination of source/destination IP addresses and ports, it can be used to implement access lists, QoS mechanisms or policy routing

To make things even more interesting, multiple TCAM entries use the same mask (don't care bits) as explained in the Understanding ACL on Catalyst 6500 Series Switches white paper. Most of that information also applies to the Catalyst 3750 platform, more details are available in the Understanding and Configuring Switching Database Manager on Catalyst 3750 Series Switches document (here is the corresponding document for Catalyst 3550). As the TCAM size on Catalyst 3750 might not be large enough, you can split it in various ways with the SDM templates.


  1. Yes, SDM is pain in the a**. I have 8540 which uses SDM and there is always not enough space for everything you need. Thank God it will be replaced with 6500 in two days (after year of struggling)
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