From my observation, spanning tree vlan mac address is the device (SUP) base mac address + (vlan_id MOD 256). To avoid duplicate (if I create vlan 506 then create vlan 250, FA is already used by vlan 506), the second lowest byte might change. Is the spanning tree mac address allocation behavior officially documented anywhere?
This is from the test on 6500
Certain 6500 supervisors have a set of 1024 MAC addresses to be used for various purposes, and they may assign consecutive MAC addresses to consecutive per-VLAN STP instances, so that every per-VLAN STP instance uses a unique Bridge ID.
Nowadays, a feature called the MAC Address Reduction is used so that instead of unique MAC addresses to create a unique Bridge ID, the VLAN ID is embedded into the lowmost 12 bits of the (formerly) configurable priority field of the Bridge ID, again yielding unique Bridge ID for every per-VLAN STP instance even with a single MAC address.
You may also want to read another thread where this feature was previously discussed:
Thanks Peter. `show catalyst6000 chassis-mac-address` is exactly what I was looking for.
I was asking because I came across people deliberately disabled system-id extension to make the switch stp root instead of adjusting priority. Is there any side effect from this kind of practice?
I came across people deliberately disabled system-id extension to make the switch stp root instead of adjusting priority.
Really? That's a very objectionable practice.
Is there any side effect from this kind of practice?
I cannot talk from a first-hand experience as I am not in charge of any 6500 Catalysts with this feature. However, these are issues I can think of: