show spanning-tree detail
Displays detailed Spanning Tree Protocol (STP) information for all VLANs or a specific VLAN, including port roles, states, timers, and bridge IDs, used for troubleshooting STP convergence and topology changes.
show spanning-tree detailWhen to Use This Command
- Verify the root bridge and port roles after a network change to ensure proper STP convergence.
- Troubleshoot a loop or connectivity issue by checking port states (e.g., blocking vs forwarding) and timers.
- Monitor topology change notifications (TCNs) to identify flapping links or unstable STP topology.
- Audit STP parameters like bridge priority, hello time, and max age for compliance with network design.
Command Examples
Show spanning-tree detail for all VLANs
show spanning-tree detailVLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0011.2233.4455
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0011.2233.4455
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/0 Desg FWD 4 128.1 P2p
Gi0/1 Desg FWD 4 128.2 P2p
Gi0/2 Desg FWD 4 128.3 P2p
Gi0/3 Desg FWD 4 128.4 P2p
VLAN0002
Spanning tree enabled protocol ieee
Root ID Priority 32770
Address 0011.2233.4455
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32770 (priority 32768 sys-id-ext 2)
Address 0011.2233.4455
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/0 Desg FWD 4 128.1 P2p
Gi0/1 Desg FWD 4 128.2 P2p
Gi0/2 Desg FWD 4 128.3 P2p
Gi0/3 Desg FWD 4 128.4 P2pVLAN0001: Shows STP is enabled with IEEE 802.1D. Root ID: priority 32769 (32768+1), MAC 0011.2233.4455, and 'This bridge is the root' indicates this switch is root. Bridge ID: same as root (since it's root). Timers: Hello 2s, Max Age 20s, Forward Delay 15s. Interface table: Gi0/0-3 are all designated (Desg) and forwarding (FWD), cost 4 (1 Gbps), port priority 128, type P2p. VLAN0002: Similar but sys-id-ext 2 (priority 32770). All ports are designated forwarding.
Show spanning-tree detail for a specific VLAN
show spanning-tree vlan 10 detailVLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 32778
Address 00a3.d1e2.f3c4
Cost 4
Port 1 (GigabitEthernet0/0)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 49162 (priority 49152 sys-id-ext 10)
Address 0011.2233.4455
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/0 Root FWD 4 128.1 P2p
Gi0/1 Altn BLK 4 128.2 P2p
Gi0/2 Desg FWD 4 128.3 P2pVLAN0010: Root ID shows priority 32778 (32768+10), MAC 00a3.d1e2.f3c4, cost 4 to root, via port Gi0/0. This switch is not root (Bridge ID different). Bridge ID priority 49162 (49152+10). Interface table: Gi0/0 is Root port (Root) and forwarding (FWD). Gi0/1 is Alternate (Altn) and blocking (BLK) — this is the backup path. Gi0/2 is Designated (Desg) and forwarding. Cost 4 for all (1 Gbps).
Understanding the Output
The output is organized per VLAN. The 'Root ID' section shows the root bridge's priority (including VLAN ID offset), MAC address, and if this switch is the root. If not root, it shows cost to root and the root port. 'Bridge ID' shows this switch's priority and MAC. Timers (Hello, Max Age, Forward Delay) are displayed. The interface table lists each port with Role (Root, Designated, Alternate, Backup), State (FWD=Forwarding, BLK=Blocking, LIS=Listening, LRN=Learning), Cost (path cost to root), Prio.Nbr (port priority and number), and Type (P2p=point-to-point, Shr=shared). Good values: Root ports and Designated ports should be FWD; Alternate/Backup should be BLK. Watch for unexpected blocking ports or high cost values that may indicate suboptimal paths. Topology changes are not shown in this command; use 'show spanning-tree detail' to see TCN counters if needed.
CCNA Exam Tips
CCNA exam tip: The root bridge is identified by the lowest bridge ID (priority + MAC). If all priorities are equal, the lowest MAC wins.
CCNA exam tip: Port roles: Root port is the best path to root; Designated port is the best path from a segment; Alternate/Backup ports are blocking.
CCNA exam tip: Port states: Blocking (20s) -> Listening (15s) -> Learning (15s) -> Forwarding. Know the timers and that Forward Delay is 15s by default.
CCNA exam tip: The 'show spanning-tree detail' command shows per-VLAN STP info. For PVST+, each VLAN has its own STP instance.
Common Mistakes
Mistake 1: Assuming all ports should be forwarding. In a stable STP topology, some ports must be blocking to prevent loops.
Mistake 2: Forgetting that STP timers (Hello, Max Age, Forward Delay) must be consistent across all switches in the same VLAN to avoid convergence issues.
Mistake 3: Misinterpreting 'This bridge is the root' — if multiple switches claim to be root, there is a misconfiguration (e.g., different VLANs or priority mismatch).
Related Commands
show spanning-tree
Displays the Spanning Tree Protocol (STP) state and configuration for all VLANs or a specific VLAN, used to verify root bridge, port roles, and STP topology.
show spanning-tree vlan [vlan-id]
Displays Spanning Tree Protocol (STP) information for a specific VLAN, including root bridge, port roles, and port states, used to verify STP topology and troubleshoot loops.
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