BCMSN Notes — STP States

I’ve decided to take on the CCNP certification, so I’m going to wind up with a few posts will be more my own notes than anything.¬† ūüôā

A switch port on a 2960 comes up with a default configuration on VLAN 1.  What happens from the perspective of spanning-tree?

  • First, the port comes up on blocking mode.¬† This is to make sure that loops aren’t created without first listening to the network to see what’s going on.
  • Next, if the port may be a root or designated port, the port is moved to the listening state.¬† In this state, the port can send and receives BPDUs only.¬† It can’t send traffic, but it can discover the other switches participating in STP.
  • After the forwarding delay, the port goes into the learning state.¬†¬† In this state, the port can send and receive BPDUs as in listening, but it can now receive traffic.¬† It can’t yet send any.
  • After the forwarding delay again, the port goes into the forwarding state.¬† The port can now send and receive data.

If the port is configured with spanning-tree portfast, the mode goes from blocking directly to forwarding without going through these steps.¬† Obviously you don’t want a switch plugged into a port configured for portfast since you may wind up with a loop.

Here’s the debug spanning-tree events output from one of my labs.¬† F0/3 is configured for portfast.¬† I shut/no shut it to see what happens.

Notice the “jump to forwarding from blocking”.

Here’s the same output when the port is not in portfast mode.¬† Notice the timestamps.¬† It takes about 30 seconds (2 x default foward delay) to go from blocking to listening to learning to forwarding.

Send any obvious corrections and questions my way.

Aaron Conaway

I shake my head around sometimes and see what falls out. That's what lands on these pages. @aconaway@mstdn.social

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