The Bridge ID and System ID Extension Explained

STP running switches elect a root bridge and use it as a focal point to create a virtual topology of the entire network. They use BID (bridge ID) to select the root bridge.

There are many variations of STP. DCE developed the original version of STP for its switches. Later, IEEE created an open-sourced version of it. It is called STP. IEEE's STP runs on all vendors' switches. But it runs only one instance on the switch. Based on IEEE's STP, Cisco developed its STP. It is called PVST+. It runs a separate instance for each VLAN. STP and PVST+ use the same terminology, algorithm, and concept to build the virtual topology and remove loops. Both use BID to elect the root bridge. Both select the switch having the lowest BID as the root bridge. However, both use a slightly different method to build a unique BID for each switch in the network.

STP uses the priority value and the switch's MAC address for the BID, while PVST+ uses the priority value, MAC address, and VLAN ID for the BID.

The default priority value is 32768 on both. It is changeable. You can configure a custom priority value in a multiple of 4096. If you want STP/PVST+ to select a particular switch as the root bridge, you can change its priority value to lower than other switches. 

Both use the switch's MAC address as the second component of the BID. Since a MAC address is a globally unique address, it makes the BID unique across the network. The MAC address in the BID is called the system ID.

PVST+ uses the VLAN ID as the third component to make the BID unique across the VLANs. Since STP does not run a separate instance for each VLAN, it does not use the VLAN ID.

The BID is 8-byte in both STP and PVST. The first two bytes store the priority value in STP and a sum of the priority value and VLAN ID in PVST+. Both use the remaining six bytes to save the MAC address or system ID.

The following image shows the BID components of STP.

stp bid

The following image shows the BID components of PVST+.

pvst bid

Changing/updating BID

Since STP/PVST uses the lowest BID as the root bridge, you can manipulate the root bridge election process by changing the BID. In a BID, only the priority value is changeable. The following global configuration mode command changes the default priority value on Cisco switches.

Router(Config)spanning-tree vlan [VLAN-ID] priority [priority value in a multiple of 4096]

change default priority

You can select a new priority value only in a multiple of 4096. If you use a value that is not a multiple of 4096, the command will not accept it and display the valid values.

In the above example, we first tried to change the priority value to 10. As expected, the command rejected the value and displayed all valid values. Later, we used the value 0, which is a valid value. The command accepted it and changed the priority value to 0.

You can use the show spanning-tree command to view the BID of the local switch and the root switch.

viewing priority

As we can see in the above output, this switch is the root bridge. The bridge priority of this switch is 1 (1 [VLAN-ID] + 0 [Custom Priority]).

Conclusion

BID is an 8-byte unique value to each switch. It contains a 2-byte priority value and a 6-byte system ID. The system ID is the MAC address of the switch. On Cisco switches, BID also contains VLAN information. It saves VLAN information with the priority value. RSPT is the updated version of STP. Both STP and RSTP use the same method for BID. RPVST is an updated version of PVST. Both RPVST and PVST use the same concept for BID.

ComputerNetworkingNotes CCNA Study Guide The Bridge ID and System ID Extension Explained