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.

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4 comments for “BCMSN Notes — STP States

  1. Iain
    May 26, 2009 at 11:39 am

    Please allow a fellow CCNP (BCMSN) seeker to vent about STP.

    STP needs to die. 6500 Multi-chassis Etherchannel (VSS) and Cross-stack etherchannel on the 3750s will probably replace STP-based designed completely. This being said, I’m sure they’ll keep STP in the cert tracks for years and years because the consultants will need it when they work on out-dated networks. Why would anyone want to use an over-complicated technology that actually blocks one of the uplinks?! Etherchannel or L3 at the access layer are technologies that actually allow the use of *both* uplinks.

    PS What out for weird wireless questions on the BCMSN test. I got like four or five questions regarding the LEDs and GUI of the relatively obscure Cisco PCMCIA wireless card (doesn’t everyone use integrated wireless now?!). The thing that really burned me was that the Official Exam Certification Guide didn’t cover this product.

  2. May 28, 2009 at 8:12 pm

    I agree with you, Iain. STP is an old design of an old technology that needs to go. Like you said, the VSS or StackWise technologies are vastly superior in that they don’t require STP to run among the cluster of switches. Is it not possible to run the same stacking technology on an Ethernet port (within reason)?

    Since these are pretty new, though, you’re right again to say that STP will be on the test for years and years.

    Thanks for posting and keep contributing!

  3. June 5, 2009 at 8:37 am

    nice explanation.

    regards
    shivlu jain

  4. Scott
    December 9, 2009 at 2:41 pm

    Nice post. To add to it, it might be helpful to insert the timing events as I’ve seen that on tests before – Blocking (20s), Listening (15s), Learning (15s) and BPDUs every 2s.

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