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CCNP Enterprise Core (350-401) Cisco Packet Forwarding

Ternary Content Addressable Memory (TCAM)

A platforms TCAM allows for the matching and evaluation of a packet on more than one field.

TCAM is an extension of the content addressable memory (CAM) architecutre but is enhanced to allow for upper layer procerssing such as identifying the Layer 2 and 3 source and destination addresses, protocol, QoS markings and more.

TCAM provides more flexibility in searching over CAM which only returns a binary result. The TCAM search provides three results on a search, 0 for true, 1 for false and X for do not care.

TCAM entries are stored in Value, Mask, and Result format.

The Value indicates the field that should be searched, examples would be the IP address and protocol fields.

The Mask indicates the field that is of interest and should be queried.

The Result is the action that should be taken with a match on the value and mask. Multiple actions can be selected along instead of just dropping or allowing traffic such as redirecting a network flow to a Quality of Service policer or specifying a pointer to a different entry in the routing table.

Many switches contain multiple TCAM entries so inbound/outbound security, quality of service, and Layer 2 and Layer 3 forwarding decisions can all take place at once.

TCAM operates on a hardware level, providing faster processing and scalability instead of processing switching.

TCAM allows some features such as access control lists (ACL) to process at the same speed regardless of whether of how big they are.

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