Cisco 2950G 24 Software Configuration Guide - Page 207
MAC Addresses and VLANs, Default MAC Address Table Configuration, Changing the Address Aging Time
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Chapter 7 Administering 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 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 shows the default MAC address table configuration. Table 7-4 Default MAC Address Table Configuration Feature Aging time Dynamic addresses Static addresses Default Setting 300 seconds Automatically learned None configured 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: Step 1 Step 2 Command configure terminal mac address-table aging-time [0 | 10-1000000] [vlan vlan-id] Step 3 end 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. 78-14982-01 Catalyst 2950 Desktop Switch Software Configuration Guide 7-55
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