MAC address table configuration
Overview
Every Ethernet switch maintains a MAC address table for forwarding frames through unicast instead of
broadcast. This table describes from which port a MAC address (or host) can be reached. When
forwarding a frame, the switch first looks up the MAC address of the frame in the MAC address table for
a match. If the switch finds an entry, it forwards the frame out of the outgoing port in the entry. If the switch
does not find an entry, it broadcasts the frame out of all but the incoming port.
How a MAC address table entry is created
The switch automatically obtains entries in the MAC address table, or you can add them manually.
MAC address learning
The switch can automatically populate its MAC address table by obtaining the source MAC addresses
(called “MAC address learning”) of incoming frames on each port.
When a frame arrives at a port, Port A for example, the switch performs the following tasks:
1. Verifies the source MAC address (for example, MAC-SOURCE) of the frame.
2. Looks up the MAC address in the MAC address table.
3. Updates an entry if it finds one. If the switch does not find an entry, it adds an entry for
MAC-SOURCE and Port A.
The switch performs the learning process each time it receives a frame from an unknown source MAC
address, until the MAC address table is fully populated.
After obtaining the source MAC address of a frame, the switch looks up the destination MAC address in
the MAC address table. If the switch finds an entry for the MAC address, it forwards the frame out of the
specific outgoing port, Port A in this example.
Manually configuring MAC address entries
With dynamic MAC address learning, a switch does not distinguish between illegitimate and legitimate
frames, which can invite security hazards. For example, if a hacker sends frames with a forged source
MAC address to a port different from the one that the real MAC address is connected to, the switch will
create an entry for the forged MAC address, and forward frames destined for the legal user to the hacker
instead.
To enhance the security of a port, you can bind specific user devices to the port by manually adding
MAC address entries into the MAC address table of the switch.
Types of MAC address table entries
A MAC address table can contain the following types of entries:
• Static entries, which are manually added and never age out.
• Dynamic entries, which can be manually added or dynamically obtained and might age out.
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