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. If you have any questions, the best way to contact me is through Twitter at @aconaway.

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