10 Replies Latest reply: Feb 29, 2012 6:58 AM by arteq RSS

    STP port states

    Keith

      Hi,

       

      I am confused with the STP port states, how long a port will transistion from Block to Forwarding, from the Cisco LAN Switching it describes a port will wait for 15 sec before going into listening state and wait another 15 sec in the learning state so that a total 0f of 30 sec.

       

      Then it go on to say that the forward delay controls both states of only 15 secs

       

       

      Cisco LAN Switching (CCIE Professional Development)

       

        • 1. Re: STP port states
          Keith

          Opps the text did not appear

           

          from the Cisco LAN Switching Book

           

          A variety of events (such as a bridge thinking it is the Root Bridge immediately after booting or an absence of BPDUs for certain period of time) can cause the bridge to transition into the Listening state. At this point, no user data is being passed—the port is sending and receiving BPDUs in an effort to determine the active topology. It is during the Listening state that the three initial convergence steps discussed in the previous section take place. Ports that lose the Designated Port election become non-Designated Ports and drop back to the Blocking state.                    Ports that remain Designated or Root Ports after 15 seconds (the default timer value) progress into the Learning state. This is another 15-second period where the bridge is still not passing user data frames

           

          Forward Delay is the time that the bridge spends in the Listening and Learning states. This is a single value that controls both states. The default value of 15 seconds

          • 2. Re: STP port states
            arteq

            consider that with plain old stp it may take anywhere from 30-50 seconds to fully transition from blocking to forwarding...  15 and 15 comprise the listening and learning to forwarding transition, which can take the whole of 30 seconds... think of basic stp as a simple yardstick measure; there are a variety of methods to reduce convergence...

            • 3. Re: STP port states
              Keith

              So the switch stay's in listening/learning for 30 seconds, The forward delay states "this is a single value which controls both states default to 15 secs"

               

              So which is it 30 secs or 15 ?

              • 4. Re: STP port states
                cadetalain

                Hi,

                 

                15s + 15 s = 30 s

                • 5. Re: STP port states
                  Keith

                  Yea but it states a forward delay controls both states of 15 seonds

                  • 6. Re: STP port states
                    Aninda

                    Not sure what the book is trying to imply, but the listening and learning states are 15 seconds EACH. So thats 30 seconds in total just like everyone has been saying so far.

                    • 7. Re: STP port states
                      cadetalain

                      Hi,

                      fwd delay is 15s and you must wait 2 X fwd delay so it makes 30 s

                      • 8. Re: STP port states
                        arteq

                        i've done this with my lab, you can actually see the transition from lis to lrn on the switch using sh spann... while each transition state may not take all of 15 seconds each, it is a good barometer...  remember that nothing is written in stone, and actual results may very depending on equipment...  30 to 50 total is the conventional wisdom with 15s each ascribed to lis and lrn states... not sure how much clearer this can be...

                        • 9. Re: STP port states
                          Keith

                          Thanks that explains it!

                          • 10. Re: STP port states
                            arteq

                            wow...