Cisco CCNA (200-301)Network AccessHard
A switch is configured with 'spanning-tree vlan 20 priority 4096'. Assuming the extended system ID feature is enabled (the default on modern Catalyst switches), what bridge priority value will actually be advertised in this switch's BPDUs for VLAN 20?
- A32768
- B4116
- C4096
- D20
Show answer & explanationAnswer & explanation
Correct answer: B. 4116
With the extended system ID enabled, the 16-bit priority field is split into a 4-bit priority multiplier (in increments of 4096) and a 12-bit VLAN ID field. When an administrator configures priority 4096 for VLAN 20, the switch adds the VLAN ID to the configured priority to form the actual advertised value: 4096 + 20 = 4116.
Why the other options are wrong
- A. 32768 is the default priority when nothing is configured, not applicable here.
- C. 4096 alone ignores the added extended system ID (VLAN number).
- D. 20 alone is just the VLAN ID, not a valid or actual priority value.
Extended System ID (Bridge Priority)
Modern Catalyst switches embed the VLAN ID into the lower bits of the bridge priority field so priority values must be multiples of 4096, with the VLAN ID added to form the actual advertised priority.
- Priority must be configured in increments of 4096 (0-61440)
- Actual advertised priority = configured priority + VLAN ID
- Default STP priority is 32768
- Lower bridge ID (priority+MAC) wins root election
Memory trick: Configured priority plus the VLAN number equals what's really sent.