Network access · Spanning tree

Spanning Tree lab:
understand, then fix.

A spanning tree lab teaches you how switches keep redundant Layer 2 links without forwarding frames around a loop. Start by predicting the root bridge, read the port roles, then correct an unexpected traffic path.

Open the free spanning tree lab

Browser simulation · No signup · Three switches and one incorrect root priority

Predict before you configure · 1/3

Who becomes root for VLAN 10?

These switches are connected in a triangle. Compare configured priority, then MAC address. Every switch participates in the same VLAN instance.

Read the election in the output

For a given VLAN, the lowest bridge ID wins. Compare priority first, then the MAC address. With the extended system ID, the priority displayed by IOS includes the VLAN number. Configured priority 4096 plus VLAN 10 appears as 4106.

SW-CORE# show spanning-tree vlan 10
VLAN0010
  Spanning tree enabled protocol rstp
  Root ID    Priority    4106
             Address     0020.0000.0001
             Cost        4
             Port        1 (GigabitEthernet0/1)
  Bridge ID  Priority    32778 (priority 32768 sys-id-ext 10)
             Address     0010.0000.0001

Interface  Role Sts Cost  Prio.Nbr Type
Gi0/1      Root FWD 4     128.1    P2p
Gi0/2      Altn BLK 4     128.2    P2p

Illustrative IOS-style output; interface IDs and costs depend on the platform and topology.

Root ID versus Bridge ID: different addresses mean this switch is not the root.
Root FWD: this is the switch’s selected path toward the root.
Altn BLK: the redundant path is not forwarding data. Blocking alone is not a fault.
Rapid STP states: discarding, learning, and forwarding. IOS may display a blocked/discarding port as BLK.

Make a prediction, change one thing, verify

In the root-bridge lab, inspect every switch before changing priority. Identify which bridge should be root, choose a priority lower than its peers, and check the election again. If priorities tie, MAC address still decides; priority zero is not a universal guarantee when another switch also uses zero.

show spanning-tree vlan 10
configure terminal
spanning-tree vlan 10 priority 0
end
show spanning-tree vlan 10

The intended root should report that it is the root. Compare port roles on the other switches, confirm the intended forwarding path, and check host reachability. An extra hop alone does not prove a latency problem; link capacity and load also matter.

Choose the right protection

PortFast: lets an edge port forward immediately. It does not disable STP.

BPDU Guard: disables a protected edge port when a BPDU arrives.

Root Guard: prevents a superior BPDU from making an unwanted downstream device the root through that port.

Loop Guard: keeps a port from incorrectly forwarding when expected BPDUs stop arriving.

BPDU Filter: suppresses BPDUs; behavior depends on how it is configured. Misuse can remove loop protection.

Put the explanation to work

Root bridge election →PortFast and BPDU Guard →Root Guard troubleshooting →STP practice questions →

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Spanning Tree lab:understand, then fix.