Manually configuring mac address entries, Types of mac address table entries, Mac address table-based frame forwarding – H3C Technologies H3C S5120 Series Switches User Manual

Page 666

1-2

updated when the aging timer expires, it is deleted. If it updates before the aging timer expires, the

aging timer restarts.

Manually configuring MAC address entries

With dynamic MAC address learning, a device does not tell illegitimate frames from legitimate ones.

This brings 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 device 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 device to bind specific user devices to the port. Because manually configured entries have

higher priority than dynamically learned ones, you can thus prevent hackers from stealing data using

forged MAC addresses.

Types of MAC Address Table Entries

A MAC address table may contain these types of entries:

Static entries, which are manually configured and never age out.

Dynamic entries, which can be manually configured 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 destination MAC addresses. For example, to block

all packets destined for a specific user for security concerns, you can configure the MAC address of

this user as a blackhole destination MAC address entry.

Dynamically-learned MAC addresses cannot overwrite static or blackhole MAC address entries, but the

latter can overwrite the former.

MAC Address Table-Based Frame Forwarding

When forwarding a frame, the device adopts the following two forwarding modes based on the MAC

address table:

Unicast mode: If an entry is available for the destination MAC address, the device forwards the

frame out the outgoing interface indicated by the MAC address table entry.

Broadcast mode: If the device receives a frame with the destination address being all ones, or no

entry is available for the destination MAC address, the device broadcasts the frame to all the

interfaces except the receiving interface.