Cisco Switching – Switch MAC Addresses Used for Bridge ID in STP

ciscomac addressspanning treeswitchswitching

Lets imagine that I have big LAN environment with many switches and many VLANs, for example, VLANs 5,15 and 50. I have PVST+ activated, and all of my switches have same (STP) priority.

I connect a new switch, with factory default settings, to my environment. And let's look only at VLAN 15. Nothing will change in this moment. Now, I create VLAN 15 on my new switch. (I didn't create a VLAN 15 SVI or add any ports to VLAN 15 – I only created VLAN 15.)

Which MAC address of this new switch will be used for the Bridge ID of VLAN 15? Will that Bridge ID be used in the Root Bridge election for VLAN 15?

Best Answer

Which MAC address of this new switch will be used for the Bridge ID of VLAN 15? Will that Bridge ID be used in the Root Bridge election for VLAN 15?

According to Cisco:

MAC Addresses for Spanning Tree Computation

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 do 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.

Each switch with that VLAN in the layer-2 domain will think it is the root bridge for that VLAN, and it will send out BPDUs with its bridge ID saying that until it receives a better BPDU for that VLAN. That means it it is used in the root bridge selection.

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