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How to change the bridge MAC address on a Cisco Catalyst 6500 series switches



The MAC addresses used for Spanning Tree calculations are stored in an EEPROM present in the Supervisor module. Regardless of the types of line modules installed, the Layer 2 MAC addresses for VLANs do not change unless you replace the Supervisor module. If you replace the Supervisor module, the Layer 2 MAC addresses of all VLANs change to those specified in the address allocator on the new Supervisor module. In the fixed configuration Catalyst switches, MAC addresses for VLANs cannot be changed. If you change the supervisor engine, this changes the mac-address used by the system and protocols like spanning-tree bridge-id.

All network interface connections (ports) require a unique MAC address. The switch system code reads the EEPROM for each interface in the system, learns the MAC addresses, and then initializes appropriate hardware and data structures. Each VLAN in the spanning tree has one unique MAC address. This addressing scheme gives the switch the intelligence to identify the state, connected or not connected, of each interface. When you hot swap a module, the MAC address changes with the module.

Usually these MAC addresses are taken from a pool, which is seen with the show module command. There are MAC addresses programmed in the backplane SPROM. These addresses are used for applications where you need MAC addresses for each VLAN, for example, the Spanning tree.

Hence, there is no command which can change the bridge mac address. In order to see the range of these MAC addresses, you can use the show catalyst6000 command.

Refer to the MAC Addresses for Spanning Tree Computation section of document Supported Platforms for Unique MAC Address Configuration on VLAN or L3 Interfaces for Catalyst Switches for more information.

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