H3C Technologies H3C S10500 Series Switches - Manual (Page 33)

22
MAC address table configuration
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 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 device 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 device performs the following tasks:
1. Verifies the source MAC address (for example, MAC-SOURCE) of the frame.
2. Looks up the source MAC address in the MAC address table.
{
If an entry is found, the device updates the entry.
{
If no entry is found, the device adds an entry for MAC-SOURCE and Port A.
3. After obtaining 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 that 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.
To improve port security and prevent hackers from stealing data using forged MAC addresses, you can
bind specific user devices to the port by manually adding MAC address entries to 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.