Mac Addresses And Vlans; Default Mac Address Table Configuration; Changing The Address Aging Time - Cisco 2950G 24 - Catalyst Switch Software Configuration Manual

Chapter 7 Administering the Switch

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 in one VLAN is unknown 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
Table 7-4
Feature
Aging time
Dynamic addresses
Static addresses

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.
Beginning in privileged EXEC mode, follow these steps to configure the dynamic address table aging
time:
Command
Step 1 configure terminal
Step 2 mac address-table aging-time [0 |
10-1000000] [vlan vlan-id]
Step 3 end
78-14982-01
shows the default MAC address table configuration.
Default MAC Address Table Configuration
Default Setting
300 seconds
Automatically learned
None configured
Purpose
Enter global configuration mode.
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.
Return to privileged EXEC mode.
Catalyst 2950 Desktop Switch Software Configuration Guide
Managing the MAC Address Table
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