The original 802.1D Spanning Tree standard defined by the IEEE only supported one unique Bridge ID per switch. This worked fine in the early days, but as VLANs were introduced Cisco needed to assign new MAC addresses for each VLAN Bridge ID as a way of staying compliant with the 802.1D standard. This quickly burned up valuable MAC addresses from the switch’s MAC table for organizations using large number of VLANs.
In response, the IEEE introduced the Extended System ID feature as a way to expand the Spanning Tree VLAN scaling support on a an individual switch. Defined by IEEE 802.1t, the Extended System ID feature allows a switch to support up to 4096 STP instances without the need to consume a separate MAC address per STP instance. This allows a single switch to use one MAC address to create all its Bridge IDs, adding the VLAN number to make each instance value unique.
To check if the Extended System ID feature is enabled on a Cisco switch, look for the presence of the spanning-tree extend system-id command in the running configuration. Most new switches use the Extended System ID feature by default and do not allow it to be removed from the configuration due to MAC address depletion concerns.
The show spanning-tree command output displays the Bridge ID information for each VLAN. Notice the Bridge ID Priority line from SW2.
SW2 is using the Extended System ID feature in the output above. If we look closer at the Bridge ID, SW2 is using the default STP priority value of 32768 but the VLAN ID of 10 is added to produce a Bridge ID priority of 32778.When the Extended System ID feature is enabled, the Bridge ID priority is the sum of the priority (default is 32768) + the System ID Extension value (sys-id-ext). The System ID Extension is just the VLAN number.