Mac Address Table Configuration; How A Mac Address Table Entry Is Created; Types Of Mac Address Table Entries - HP 5820X series Configuration Manual

MAC address table configuration

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 an entry is found, the switch forwards the frame out of the outgoing port in the entry. If no
entry is found, the switch broadcasts the frame out of all but the incoming port.

How a MAC address table entry is created

The entries in the MAC address table come from two sources: automatically learned by the switch and
manually added by the administrator.
MAC address learning
The switch can automatically populate its MAC address table by learning the source MAC addresses of
incoming frames on each port.
When a frame arrives at a port, Port A for example, the switch performs the following tasks:
Checks the source MAC address (MAC-SOURCE for example) of the frame.
1.
Looks up the MAC address in the MAC address table.
2.
If an entry is found, updates the entry. If no entry is found, adds an entry for MAC-SOURCE and
3.
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 learning the source MAC address of a frame, the switch looks up the destination MAC address in
the MAC address table. If an entry is found for the MAC address, the switch 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 where 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 manually add MAC address entries into the MAC address
table of the switch to bind specific user devices to the port.

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 learned and may age out.
•
Blackhole entries, which are manually configured and never age out. Blackhole entries are
•
configured for filtering out frames with specific source or destination MAC addresses. For example,
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