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Configuring the MAC address table
Overview
An Ethernet device uses a MAC address table to forward frames. A MAC address entry includes a
destination MAC address, an outgoing interface, and a VLAN ID. When the device receives a frame,
it uses the destination MAC address of the frame to look for a match in the MAC address table.
•
The device forwards the frame out of the outgoing interface in the matching entry if a match is
found.
•
The device floods the frame in the VLAN of the frame if no match is found.
How a MAC address entry is created
The entries in the MAC address table include entries automatically learned by the device and entries
manually added.
MAC address learning
The device can automatically populate its MAC address table by learning the source MAC addresses
of incoming frames on each interface.
When a frame arrives at an interface (for example, port A), the device performs the following tasks:
1.
Checks the source MAC address (for example, MAC-SOURCE) of the frame.
2.
Looks up the source MAC address in the MAC address table.
{
The device updates the entry if an entry is found.
{
The device adds an entry for MAC-SOURCE and port A if no entry is found.
3.
When the device receives a frame destined for MAC-SOURCE after learning this source MAC
address, the device performs the following tasks:
a.
Finds the MAC-SOURCE entry in the MAC address table.
b.
Forwards the frame out of port A.
The device performs the learning process each time it receives a frame with an unknown source
MAC address until the table is fully populated.
Manually configuring MAC address entries
Dynamic MAC address learning does not distinguish between illegitimate and legitimate frames,
which can invite security hazards. When Host A is connected to port A, a MAC address entry will be
learned for the MAC address of Host A (for example, MAC A). When an illegal user sends frames
with MAC A as the source MAC address to port B, the device performs the following tasks:
1.
Learns a new MAC address entry with port B as the outgoing interface and overwrites the old
entry for MAC A.
2.
Forwards frames destined for MAC A out of port B to the illegal user.
As a result, the illegal user obtains the data of Host A. To improve the security for Host A, manually
configure a static entry to bind Host A to port A. Then, the frames destined for Host A are always sent
out of port A. Other hosts using the forged MAC address of Host A cannot obtain the frames destined
for Host A.
Types of MAC address entries
A MAC address table can contain the following types of entries:
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