H3C Technologies H3C SecBlade NetStream Cards - Manual (Page 123)

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MAC address table configuration
This chapter includes these sections:
• Overview
• Configuring the MAC address table
• Displaying and maintaining MAC address tables
• MAC address table configuration example
Overview
An Ethernet device uses 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 device first looks up the MAC address of the frame in the MAC address table for
a match. If an entry is found, the device forwards the frame out of the outgoing port in the entry. If no
entry is found, the device 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 device and
manually added by the administrator.
MAC address learning
The device 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 device performs the following tasks:
1. Checks the source MAC address (MAC-SOURCE for example) of the frame.
2. Looks up the source MAC address in the MAC address table.
a. If an entry is found, the device updates the entry.
b. If no entry is found, the device adds an entry for MAC-SOURCE and Port A.
3. After learning this source MAC address, when the device receives a frame destined for
MAC-SOURCE, the device finds the MAC-SOURCE entry in the MAC address table and forwards
the frame out Port A.
The device performs the learning process each time it receives a frame from an unknown source MAC
address, until the MAC address table is fully populated.
Manually configuring MAC address entries
With dynamic MAC address learning, a device does not distinguish between illegitimate and legitimate
frames, which can invite security hazards. For example, when a hacker sends frames with a forged
source MAC address to a port different from the one where the real MAC address is connected, the
device creates an entry for the forged MAC address, and forwards frames destined for the legal user to
the hacker instead.