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Spanning tree

How to find which switch is root bridge for each VLAN

Spanning tree decides which links forward and which stand by, and it decides it from the root bridge outward. If the root is not where you designed it, traffic takes a path you did not choose. The STP listing shows, for every switch and every VLAN, which bridge it currently believes is root, so you can check the whole estate against your design in one read.

All the questions
Before you start

What you need

  • Module required: stp.
  • Permission required: view_stp.
  • Cisco and Huawei switches only

1. Open L2 Services then Spanning-Tree

app.nactrack.com/spanning-tree
Open L2 Services then Spanning-Tree
Open L2 Services and choose Spanning-Tree. The upper table is one row per switch per VLAN: Bridge ID is that switch's own identity, and Root Bridge is the switch it currently believes is root. Both are shown as address plus hostname, so you never have to translate a MAC.

2. Read a domain

app.nactrack.com/spanning-tree
Read a domain
Read down the Root Bridge column. Every VLAN on a healthy campus should point at the same aggregation switch, and Root Priority tells you whether that was configured on purpose or won by accident on a tie-breaking address.
The limits

What this view does not tell you

  • A priority left at the default means the root was decided by the lowest MAC address, not by design. It will move the day that switch is replaced.
  • Spanning tree is collected from Cisco and Huawei switches only, and is stripped from routers. An Aruba or Dell access layer will not appear here even though it is running spanning tree.

Try the same thing on your own estate

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