What is the Source of the MAC Address Used in a BID (Bridge ID)?

[promanelka]

For STP to work properly, each switch needs a BID which it gets by combining a 2-byte Priority Field and a 6-byte MAC Address.

Consider

A switch can have 24 ethernet ports, each with a Network Interface Card (NIC) having a unique MAC address. Which of these MAC addresses then is used? None. To ensure a unique BID, switches use a burned-in MAC address from the switch's base system (backplane). It is this base system MAC when combined with the Priority Field that makes the BID.

So, even if a switch has multiple interfaces, it does not use an individual port's MAC address for the BID.

@Ramblin Tech M02@rt37 

[MHM Cisco World]

Sorry what is your Q? Can you elaborate 

MHM

[balaji.bandi]

MAC address of the switch.

check some learning concepts from below blog :

https://www.firewall.cx/networking/network-protocols/spanning-tree-protocol/spanning-tree-protocol-root-bridge-election.html

[Ramblin Tech · Accepted solution]

The 64-bit Bridge ID is comprised from the concatenation of the 4-bit priority field + 12-bit MSTI/CIST ID + 48-bit Bridge Address. Your question centers on the origin of the Bridge Address

From 802.1Q-2022 8.13.8 Unique identification of a Bridge:

"A unique EUI-48 Universally Administered MAC address, termed the Bridge Address, shall be assigned to
each Bridge. The Bridge Address may be the individual MAC address of a Bridge Port; in which case, use of
the address of the lowest numbered Bridge Port (Port 1) is recommended.
NOTE—RSTP (Clause 13) and MSTP (Clause 13) require that a single unique identifier be associated with each Bridge.
That identifier is derived from the Bridge Address as specified in 14.2.5."

The Bridge Address may be an address of a bridge port, but is not required to be; it need only be a globally unique address. If it does use the MAC address of a bridge port, the standard recommends using the lowest numbered address.