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Catalyst2950 Desktop Switch Software Configuration Guide
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Chapter7 Administering the Swi tc h Managing the MAC Address Table
MAC Addresses and VLANs
All addresses are associated with a VLAN. An address can exist in more than one VLAN and have
different destinations in each. Multicast addresses, for example, could be forwarded to port 1 in VLAN 1
and ports 9, 10, and 11 in VLAN 5.
Each VLAN maintains its own logical address table. A known address i n one V LA N i s u nknown in
another until it is learned or statically associated with a port in the other VLAN. Addresses that are
statically entered in one VLAN must be configured as static addresses in all other VLANs or remain
unlearned in the other VLANs.
Default MAC Address Table Configuration
Table 7 -4 shows the default MAC address table configuration.
Changing the Address Aging Time
Dynamic addresses are source MAC addresses that the switch learn s a nd th en ag es wh en th e y ar e n ot i n
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 unkno wn desti nation, it floods the p acket to all ports in the same
VLAN as the receiving port. This unnecessary flooding can imp act per for manc e. Set ti ng t oo 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.
Beginning in privileged EXEC mode, follow these steps to configure the dynamic address table aging
time:
Table7-4 Default MAC Address Table Configuration
Feature Default Setting
Aging time 300 seconds
Dynamic addresses Automatically learned
Static addresses None configured
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 mac address-table aging-time [0 |
10-1000000] [vlan vlan-id]Set the length of time that a dynamic entry remains in the MAC
address table after the entry is used or updated.
The range is 10 to 1000000 seconds. The default is 300. You can also
enter 0, which disables aging. Static address entries are never aged
or removed from the table.
For vlan-id, valid IDs are 1 to 4094 when the enhanced software
image (EI) is installed and 1 to 1005 when the standard software
image (SI) is installed. Do not enter leading zeros.
Step3 end Return to privileged EXEC mode.