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Catalyst 3750 Switch Software Configuration Guide
OL-8550-02
Chapter 7 Administering the Switch
Managing the MAC Address Table
When private VLANs are configured, address learning depends on the type of MAC address:
• Dynamic MAC addresses learned in one VLAN of a private VLAN are replicated in the associated
VLANs. For example, a MAC address learned in a private-VLAN secondary VLAN is replicated in
the primary VLAN.
• Static MAC addresses configured in a primary or secondary VLAN are not replicated in the
associated VLANs. When you configure a static MAC address in a private VLAN primary or
secondary VLAN, you should also configure the same static MAC address in all associated VLANs.
For more information about private VLANs, see Chapter 16, “Configuring Private VLANs.”
MAC Addresses and Switch Stacks
The MAC address tables on all stack members are synchronized. At any given time, each stack member
has the same copy of the address tables for each VLAN. When an address ages out, the address is
removed from the address tables on all stack members. When a switch joins a switch stack, that switch
receives the addresses for each VLAN learned on the other stack members. When a stack member leaves
the switch stack, the remaining stack members age out or remove all addresses learned by the former
stack member.
Default MAC Address Table Configuration
Table 7-3 shows the default MAC address table configuration.
Changing the Address Aging Time
Dynamic addresses are source MAC addresses that the switch learns and then ages when they are not in
use. You can change the aging time setting for all VLANs or for a specified VLAN.
Setting too short an aging time can cause addresses to be prematurely removed from the table. Then
when the switch receives a packet for an unknown destination, it floods the packet to all ports in the same
VLAN as the receiving port. This unnecessary flooding can impact performance. Setting too long an
aging time can cause the address table to be filled with unused addresses, which prevents new addresses
from being learned. Flooding results, which can impact switch performance.
Table 7-3 Default MAC Address Table Configuration
Feature Default Setting
Aging time 300 seconds
Dynamic addresses Automatically learned
Static addresses None configured
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