How Does TCAM Work in Routers?

How Does TCAM Work in Routers?

In a previous post, I asked if TCAM is still needed in modern routers. In this post, I will examine how TCAM works in routers and provide some more insight into TCAM.

In a router, TCAM stores and processes rules for various functions. These rules are typically derived from:

  • Routing tables are used to determine the best path for packets.
  • Access Control Lists (ACLs) for enforcing security policies.
  • QoS policies for prioritizing traffic.

Steps for TCAM Lookup:

  1. A router receives a packet and extracts key fields like the destination IP address, protocol, or port number.
  2. These fields are compared against the rules stored in the TCAM.
  3. TCAM identifies the best match in a single clock cycle and returns the corresponding action (e.g., forward, drop, prioritize).

Why Do Routers Use TCAM?

TCAM is integral to routers because of its unique capabilities:

  1. Speed: TCAM’s ability to perform parallel searches enables routers to process millions of packets per second. This is extremely helpful in routers pulling in full BGP routing tables.
  2. Wildcard Matching: The ternary logic allows for flexible rules, such as matching a range of IP addresses or protocol types.
  3. Scalability: TCAM supports thousands of rules, making it suitable for large networks with complex configurations.

Use Cases of TCAM in Routers

1. Routing Table Lookups
Routers use TCAM to perform lookups in the Forwarding Information Base (FIB). TCAM ensures quick matching of destination IP addresses against the routing table, enabling high-speed packet forwarding.

2. Access Control Lists (ACLs)
ACLs define rules for allowing or blocking traffic. TCAM allows routers to evaluate incoming packets against hundreds or thousands of ACL entries in real time.

3. Quality of Service (QoS)
QoS policies prioritize certain types of traffic (e.g., VoIP or video) over others. TCAM helps classify packets and enforce these policies efficiently.

4. Network Address Translation (NAT)
In NAT operations, TCAM can quickly match packets to translation rules, ensuring seamless communication between private and public networks.

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