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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 7 Adminis tering the Switch 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 i n VLAN 5.
Each V LA N ma int ain s its own log ica l a ddre ss ta ble. A known a ddress 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
unlearne d in the other VL ANs.

Default MAC Address Table Configuration

Table 7-4 shows the default MAC address table configurati on.

Changing the Address Aging Time

Dynami c add re s ses ar e so ur ce MAC add re s ses tha t th e s witc h learn s and then 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 t he switch re ceiv es a pa cket for an unkno wn desti nation, it floods the p acket to all ports in the same
VLAN a s t he re ceiving po rt. Th is un ne cessa ry floo ding c an i mp act per for manc e. Set ting t oo lo ng an
aging time can cau se the addre ss table to be f illed with unused addresses, wh ich pre v en ts ne w addresses
from being l earned . Flooding results, which ca n impac t switch perf ormance .
Beginning in privileged EX EC mode, fol low these steps to con figure the dynam ic addre ss table agin g
time:
Table 7-4 Default MAC Address Table Configuration
Feature Default Setting
Agin g time 300 secon ds
Dyna mic addr esses Auto matically learned
Stati c addresse s None co nfigured
Command Purpose
Step 1 configure terminal Enter g l obal c onfigura tion m od e.
Step 2 mac address-table aging-time [0 |
10-1000000] [vlan vlan-i d]Set the length of time that a dynamic entry remains in the MAC
addre ss table after the entry is use d 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
imag e (EI) is installed and 1 to 1005 whe n the stan dard software
image (SI) is installed. Do not enter leading zeros.
Step 3 end Return to privileged EXEC mode.