What does a switch search for in the CAM table when forwarding a frame?

When a Layer 2 switch receives an Ethernet frame, it examines the frame's destination

MAC address. The switch then searches its Content Addressable Memory (CAM) table

(also known as the MAC address table) for an entry corresponding to this destination MAC

address. If a match is found, the CAM table entry provides the specific switch port

(destination port) out of which the frame should be forwarded to reach its destination. Thus,

the switch is effectively looking for the mapping between the destination MAC address and

its associated egress port.

A: source MAC address and aging time: The source MAC address is used to learn and

populate the CAM table, not as the primary key for forwarding lookups. Aging time is for

CAM entry maintenance.

B: destination MAC address and flush time: While the destination MAC address is the

search key, "flush time" is not a parameter searched for; aging mechanisms lead to flushing

old entries.

C: source MAC address and source port: These are used by the switch during the MAC

address learning process to populate the CAM table, not for the forwarding decision of an

existing frame to its destination.

1. Odom, W. (2019). CCNA 200-301 Official Cert Guide, Volume 1. Cisco Press.

Chapter 2, "Ethernet LAN Switching Concepts," Section: "Building the MAC Address Table"

and "Forwarding Known Unicast Frames." Specifically, "When a switch receives a frame,

the switch looks at the frame-™ s destination MAC address. The switch then looks for that

same MAC address in its MAC address table. If the destination MAC address is in the table,

the switch sends the frame out the one port listed in the table." (Page 59, Pearson eText).

This confirms the switch searches for the destination MAC address to find the port.

2. Cisco. (n.d.). LAN Switching Fundamentals Part 1 (Self-Study). Cisco Learning Network.

/561

Module: "Switch Operation." This resource typically explains that switches use the

destination MAC address to look up the egress port in the MAC address table. For example,

"When a frame arrives at a switch port, the switch looks in the MAC address table for the

destination MAC address." (Content available through Cisco Learning Network

subscriptions or related Cisco documentation).

3. IEEE Std 802.1D-2004. (2004). IEEE Standard for Local and metropolitan area networks

Media Access Control (MAC) Bridges. IEEE.

Section 7.8, "The Forwarding Process," describes that the bridge (switch) uses the

destination MAC address to search its Filtering Database (CAM table) to determine the

outgoing port. "If the destination MAC Address is found in the Filtering Database...the frame

shall be forwarded on the Port identified in the database." (Page 60).

/561