Introduction to MAC
Entries in a MAC
This chapter describes the management of static, dynamic, and blackhole MAC
address entries. For information about the management of multicast MAC address
entries, refer to Chapter 29.
A MAC address table is a port-based Layer 2 address table. It is the base for Ethernet
switch to perform Layer 2 packet forwarding. Each entry in a MAC address table
contains the following fields:
Destination MAC address
ID of the VLAN which a port belongs to.
Forwarding port number.
Upon receiving a packet, a switch queries its MAC address table for the forwarding
port number according to the destination MAC address carried in the packet and
then forwards the packet through the port.
Entries in a MAC address table fall into the following categories according to their
characteristics and configuration methods:
Static MAC address entry: Also known as permanent MAC address entry. This type
of MAC address entries are added/removed manually and can not age out by
themselves. Using static MAC address entries can reduce broadcast packets
remarkably and are suitable for networks where network devices seldom change.
Dynamic MAC address entry: This type of MAC address entries are generated by
the MAC address learning mechanism and age out after the aging time.
Blackhole MAC address entry: This type of MAC address entries are configured
manually. A switch discards the packets destined for the MAC addresses contained
in blackhole MAC address entries.
Table 69 lists the different types of MAC address entries and their characteristics.
Table 69 Characteristics of different types of MAC address entries
MAC address entry
Static MAC address
Dynamic MAC address
Blackhole MAC address
Manually configured or
generated by MAC
Reserved or not at
reboot (if the
configuration is saved)