The Extended System ID is the VLAN ID included in a spanning-tree bridge priority. In Cisco’s implementation, the bridge priority combines a configurable priority value with the Extended System ID. For example, the default value 32768 becomes 32769 for VLAN 1 and 32770 for VLAN 2. section 14.3.1
This value matters during root bridge election because the lowest bridge ID wins. Since each VLAN contributes its own ID, Cisco PVST+ creates a separate bridge priority and spanning-tree instance for each VLAN. Different VLANs can therefore block different links and distribute traffic across the network. section 14.3.1 section 14.6
Reading the Extended System ID in CLI output
The command output `Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)` shows a configured priority of 32768 and an Extended System ID of 1 for VLAN 1. The resulting bridge priority is 32769. For VLAN 2, the same configured value would produce 32770. section 14.3.1
Apply the concept when controlling spanning-tree elections:
- Compare the complete bridge ID, not only the configured priority; the VLAN ID is added to that priority. section 14.3.1
- Use `show spanning-tree` to view the root bridge’s BID, the local BID, and the displayed system ID extension. section 14.6
- Configure the base priority in increments of 4096 with `spanning-tree vlan vlan-id priority priority-value`; the VLAN ID remains part of the resulting priority. section 14.6
