show spanning-tree brief
Displays a summary of Spanning Tree Protocol (STP) information for all VLANs, showing the root bridge, port states, and roles in a compact format, used to quickly verify STP topology and identify blocked ports.
show spanning-tree briefWhen to Use This Command
- After configuring STP, verify that the expected root bridge is elected and that redundant links are properly blocking.
- Troubleshoot a network loop by checking for unexpected port roles or states.
- Quickly assess STP status across multiple VLANs when deploying new switches.
- Monitor STP convergence after a link failure or topology change.
Command Examples
Basic STP brief output for all VLANs
show spanning-tree briefVLAN Root ID Cost Port ---- ------------------------------ ------ ---- 1 32769.aaaa.bbbb.cccc 4 Gi0/1 1 32769.aaaa.bbbb.cccc 4 Gi0/2 1 32769.aaaa.bbbb.cccc 4 Gi0/3 1 32769.aaaa.bbbb.cccc 4 Gi0/4 1 32769.aaaa.bbbb.cccc 4 Gi0/5 1 32769.aaaa.bbbb.cccc 4 Gi0/6 1 32769.aaaa.bbbb.cccc 4 Gi0/7 1 32769.aaaa.bbbb.cccc 4 Gi0/8 1 32769.aaaa.bbbb.cccc 4 Gi0/9 1 32769.aaaa.bbbb.cccc 4 Gi0/10 1 32769.aaaa.bbbb.cccc 4 Gi0/11 1 32769.aaaa.bbbb.cccc 4 Gi0/12 1 32769.aaaa.bbbb.cccc 4 Gi0/13 1 32769.aaaa.bbbb.cccc 4 Gi0/14 1 32769.aaaa.bbbb.cccc 4 Gi0/15 1 32769.aaaa.bbbb.cccc 4 Gi0/16 1 32769.aaaa.bbbb.cccc 4 Gi0/17 1 32769.aaaa.bbbb.cccc 4 Gi0/18 1 32769.aaaa.bbbb.cccc 4 Gi0/19 1 32769.aaaa.bbbb.cccc 4 Gi0/20 1 32769.aaaa.bbbb.cccc 4 Gi0/21 1 32769.aaaa.bbbb.cccc 4 Gi0/22 1 32769.aaaa.bbbb.cccc 4 Gi0/23 1 32769.aaaa.bbbb.cccc 4 Gi0/24
VLAN: VLAN number. Root ID: Bridge priority and MAC address of the root bridge (32769.aaaa.bbbb.cccc). Cost: Path cost to the root bridge (4 indicates a 10 Gbps link). Port: Local port. All ports show the same root ID, meaning this switch is not the root (the root would show its own MAC). All ports are in forwarding state (no 'BLK' or 'ALT' shown).
STP brief with alternate/blocked ports
show spanning-tree briefVLAN Root ID Cost Port ---- ------------------------------ ------ ---- 1 32769.aaaa.bbbb.cccc 4 Gi0/1 1 32769.aaaa.bbbb.cccc 4 Gi0/2 1 32769.aaaa.bbbb.cccc 4 Gi0/3 1 32769.aaaa.bbbb.cccc 4 Gi0/4 1 32769.aaaa.bbbb.cccc 4 Gi0/5 1 32769.aaaa.bbbb.cccc 4 Gi0/6 1 32769.aaaa.bbbb.cccc 4 Gi0/7 1 32769.aaaa.bbbb.cccc 4 Gi0/8 1 32769.aaaa.bbbb.cccc 4 Gi0/9 1 32769.aaaa.bbbb.cccc 4 Gi0/10 1 32769.aaaa.bbbb.cccc 4 Gi0/11 1 32769.aaaa.bbbb.cccc 4 Gi0/12 1 32769.aaaa.bbbb.cccc 4 Gi0/13 1 32769.aaaa.bbbb.cccc 4 Gi0/14 1 32769.aaaa.bbbb.cccc 4 Gi0/15 1 32769.aaaa.bbbb.cccc 4 Gi0/16 1 32769.aaaa.bbbb.cccc 4 Gi0/17 1 32769.aaaa.bbbb.cccc 4 Gi0/18 1 32769.aaaa.bbbb.cccc 4 Gi0/19 1 32769.aaaa.bbbb.cccc 4 Gi0/20 1 32769.aaaa.bbbb.cccc 4 Gi0/21 1 32769.aaaa.bbbb.cccc 4 Gi0/22 1 32769.aaaa.bbbb.cccc 4 Gi0/23 1 32769.aaaa.bbbb.cccc 4 Gi0/24
This output shows all ports in forwarding state. In a real network with redundancy, some ports would show 'ALT' (alternate) or 'BLK' (blocked). The absence of such indicates either no redundant links or all ports are forwarding (possible loop if multiple paths exist).
Understanding the Output
The 'show spanning-tree brief' command outputs a table with columns: VLAN (VLAN ID), Root ID (bridge priority and MAC address of the root bridge for that VLAN), Cost (path cost from this switch to the root bridge), and Port (local interface). The Root ID format is 'priority.mac-address'. A lower priority value (default 32768) indicates a better candidate for root bridge. The cost value depends on link speed: 4 for 10 Gbps, 19 for 100 Mbps, 100 for 10 Mbps. If a port is in blocking or alternate role, it will be indicated in the Port column (e.g., 'Gi0/1 BLK' or 'Gi0/2 ALT'). In the basic output, only forwarding ports are listed; blocked ports are omitted unless explicitly shown. To see all ports including blocked, use 'show spanning-tree' without 'brief'. In a well-designed network, you should see exactly one forwarding port per redundant link and the rest blocking. If all ports are forwarding on a switch with multiple uplinks, a loop may exist.
CCNA Exam Tips
CCNA exam often tests that the root bridge is the switch with the lowest bridge ID (priority + MAC). Use 'show spanning-tree brief' to quickly identify the root by looking at the Root ID column.
Remember that 'brief' output only shows forwarding ports by default; if you need to see blocked ports, use 'show spanning-tree' without 'brief'.
The path cost in the output is the cost to reach the root bridge, not the cost of the local link. For example, a cost of 19 means the path to the root includes a 100 Mbps link.
Be aware that PVST+ runs per VLAN; the root bridge can differ per VLAN. 'show spanning-tree brief' shows all VLANs; look for different Root IDs across VLANs.
Common Mistakes
Assuming all ports in the output are forwarding; the brief output may omit blocked ports. Always use 'show spanning-tree' to see full port states.
Misinterpreting the cost column as the cost of the local port instead of the cumulative cost to the root bridge.
Forgetting that the root bridge itself will show its own MAC in the Root ID column and a cost of 0 (though brief may not show cost 0 explicitly).
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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