HP Compaq Presario,Presario 1910, Руководство пользователя

Страница: 198 / 578

185
Configuring MAC address tables
MAC address configurations related to interfaces apply only to Layer 2 Ethernet interfaces.
This document covers only the management of unicast MAC address entries, including static, dynamic,
and blackhole MAC address entries.
Overview
To reduce single-destination packet floodings in a switched LAN, an Ethernet device uses a MAC
address table to forward frames. This table describes from which port a MAC address (or host) can be
reached. Upon receiving a frame, the device uses the destination MAC address of the frame to look for
a match in the MAC address table. If a match is found, the device forwards the frame out of the outgoing
interface in the matching entry. If no match is found, the device floods the frame out of all but the
incoming port.
How a MAC address table entry is created
The device automatically learns 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 learning the source MAC addresses of
incoming frames on each port.
When a frame arrives at a port (for example, Port A), 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. When the device receives a frame destined for MAC-SOURCE after learning this source MAC
address, the device finds the MAC-SOURCE entry in the MAC address table and forwards the
frame out of Port A.
The device performs this 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. For example, when a hacker sends frames with a forged source MAC address to a port different
from the one with which the real MAC address is associated, 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, you can manually add MAC address entries to the MAC address table of the
device to bind specific user devices to the port.