NacTrack
FRGet the demo
Spanning tree

How to find which ports spanning tree is blocking

A blocking port is not a fault. It is spanning tree doing its job, holding a redundant link in reserve so the network has no loop. What matters is whether the ports it chose to block are the ones you would have chosen, because a link blocked at the wrong end sends traffic the long way round every day without anyone noticing.

All the questions
Before you start

What you need

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

1. Open the STP listing

app.nactrack.com/spanning-tree
Open the STP listing
Open Spanning-Tree then STP Listing. The lower table is the one we want: every port taking part in spanning tree, on every switch, with the role it was given and the state it settled into.

2. Read role and state

app.nactrack.com/spanning-tree
Read role and state
Two columns carry the answer. Role is the job spanning tree gave the port, and State is what it is actually doing. A port with role Alternate and state Blocking is a healthy standby: it has a path to the root but a better one exists, so it waits.
The limits

What this view does not tell you

  • Most ports on an access switch read Disabled rather than Forwarding or Blocking. That is not spanning tree rejecting them; it is simply that nothing is plugged in.
  • This is the state at the last collection, not a live feed. A port that has since converged differently will still read as it did when it was collected.

Try the same thing on your own estate

Request accessAll the questionsEvery guide as a PDF (25 MB)