The MAC Address Table

Explain how a switch builds its MAC address table and forwards frames.

Switch Fundamentals

• A Layer 2 Ethernet switch uses Layer 2 MAC addresses to make forwarding decisions.
• It is completely unaware of the data (protocol) being carried in the data portion of the frame, such as an IPv4 packet, an ARP message, or an IPv6 ND packet.
• The switch makes its forwarding decisions based solely on the Layer 2 Ethernet MAC addresses.
• An Ethernet switch examines its MAC address table to make a forwarding decision for each frame, unlike legacy Ethernet hubs that repeat bits out all ports except the incoming port.
• When a switch is turned on, the MAC address table is empty

Note: The MAC address table is sometimes referred to as a content addressable memory (CAM) table.

Switch Learning and Forwarding

Examine the Source MAC Address (Learn)

• Every frame that enters a switch is checked for new information to learn.
• It does this by examining the source MAC address of the frame and the port number where the frame entered the switch.
• If the source MAC address does not exist, it is added to the table along with the incoming port number.
• If the source MAC address does exist, the switch updates the refresh timer for that entry.
• By default, most Ethernet switches keep an entry in the table for 5 minutes.

Note: If the source MAC address does exist in the table but on a different port, the switch treats this as a new entry. The entry is replaced using the same MAC address but with the more current port number.

Find the Destination MAC Address (Forward)

• If the destination MAC address is a unicast address, the switch will look for a match between the destination MAC address of the frame and an entry in its MAC address table.
• If the destination MAC address is in the table, it will forward the frame out the specified port.
• If the destination MAC address is not in the table, the switch will forward the frame out all ports except the incoming port. This is called an unknown unicast.

Note: If the destination MAC address is a broadcast or a multicast, the frame is also flooded out all ports except the incoming port.

Filtering Frames

• As a switch receives frames from different devices, it is able to populate its MAC address table by examining the source MAC address of every frame.
• When the MAC address table of the switch contains the destination MAC address, it is able to filter the frame and forward out a single port.

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