11 Replies Latest reply: Jan 31, 2012 6:34 AM by Elvin Arias RSS

    Spanning Tree


      I am testing spanning tree using Cisco 2960. The default spanning tree in 2960 is PVST+

      As my understanding PVST+ has only three port roles (Please correct me if I am wrong)

      1. Root port

      2. Designate port

      3. Non-Designate port


      Can someone explain why when I show spanning-tree in 2960

      It has Altn port as it's port role (I connect three 2960 to form a loop to do this test)

      I have verify that the current spanning tree is PVST as below.

      Does Altn port role only happen in Rapid-Spanning Tree not PVST


      S2#sh spanning-tree
        Spanning tree enabled protocol ieee
        Root ID    Priority    32769
                   Address     0060.4719.807B
                   Cost        19
                   Port        2(FastEthernet0/2)
                   Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

        Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
                   Address     00E0.B084.6EAC
                   Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
                   Aging Time  20

      Interface        Role Sts Cost      Prio.Nbr Type
      ---------------- ---- --- --------- -------- --------------------------------
      Fa0/2            Root FWD 19        128.2    P2p
      Fa0/3           Altn BLK 19        128.3    P2p



      S2#sh spanning-tree summary
      Switch is in pvst mode
      Root bridge for:
      Extended system ID           is enabled
      Portfast Default             is disabled
      PortFast BPDU Guard Default  is disabled
      Portfast BPDU Filter Default is disabled
      Loopguard Default            is disabled
      EtherChannel misconfig guard is disabled
      UplinkFast                   is disabled
      BackboneFast                 is disabled
      Configured Pathcost method used is short

      Name                   Blocking Listening Learning Forwarding STP Active
      ---------------------- -------- --------- -------- ---------- ----------
      VLAN0001                     1         0        0          1          2

      ---------------------- -------- --------- -------- ---------- ----------
      1 vlans                      1         0        0          1          2

        • 1. Re: Spanning Tree
          Kumar Mantri

          yes mira,


          The  RSTP has ROOT,DESIGNATED,ALTERNATE and BACKUP where in PVST has Root,designated and non-designated.

          • 2. Re: Spanning Tree
            Scott Morris - CCDE/4xCCIE/2xJNCIE

            There is also Rapid PVST.  Which incorporates the stuff from 802.1w along with PVST+.



            • 3. Re: Spanning Tree

              Hi Scott,

              Yes, you are right Rapid-PVST can be config on 2960 with the command

              spanning-tree mode rapid-pvst.

              My question was when I use PVST why the port role is Altn which should only exist in rapid spanning-tree not PVST

              Any idea?

              Thanks, Mira

              • 4. Re: Spanning Tree

                Hi Mira.


                Cisco changed this. There is a discussion in cisco support forum:




                "It is true that the port roles were introduced with RSTP. However, they definitely make sense for STP (for instance, the difference between an alternate and a backup port is useful to uplinkfast, and it's also nice to see clearly what the root port is) so we decided to display the role in STP mode also (something we did not do for CatOS afaik)."

                • 5. Re: Spanning Tree

                  OK. Let me summarize this. Please correct me if I am wrong.

                  When Cisco switch run PVST/PVST+ they use Rapid-STP port role even they are not Rapid.

                  One thing I am still not clear is

                  " the difference between an alternate and a backup port is useful to uplinkfast, and it's also nice to see clearly what the root port is"


                  Could you please explain this?



                  • 6. Re: Spanning Tree

                    There is no non-designated port role anymore . Just Alternate, Designated, Root.

                    • 7. Re: Spanning Tree
                      Elvin Arias

                      I'll explain.


                      In 802.1w/RSTP was optimize for better progression through the STP states reducing convergence time. The rapidness of 802.1w/RSTP is totally dependent of the synchronization process. The backup port role in 802.1w/RSTP can be seen when you have a redundant port connected to a shared segment, what does this mean? This means that the port will not have the "rapid" convergence that 802.1w/RSTP normally has (will not perform the sync process).


                      RSTP have the UplinkFast mechanisms built-in by default (eventhough how is implemented migh not be techically the same as Cisco propietary UplinkFast found in PVST/PVST+) it has the same basic effect "switchover to the alternate port to reach the root as fast as possible".


                      The synchronization process can't be perform on a shared link, and that's the place where you will likely find a backup port that could be what that sentence means.





                      • 8. Re: Spanning Tree

                        I am also confused by this.

                        Has PVST/802.1d adopted RSTP/802.1w port roles?


                        If it has, then what really differentiates the two other than RSTP not having a "listening" state?

                        • 9. Re: Spanning Tree
                          Elvin Arias

                          802.1w/RSTP grouped the port states, because there is not difference from a non-forwarding perspective point of view, so for that reason they took these and made one called discarding which grouped the disable, blocking, and listening state in the case of the learning and forwarding state the states are basically equal. See this earlier post where that is explained.  https://learningnetwork.cisco.com/message/199721#199721


                          Post any doubts,



                          • 10. Re: Spanning Tree

                            Thanks very much Elvin.

                            My last doubt is this:

                            802.1w/RSTP defined the Alternate port state, correct?

                            Am I to understand that 802.1d/STP adopted this port state?


                            Thanks again, and sorry for being all confused!

                            • 11. Re: Spanning Tree
                              Elvin Arias

                              Firstable, is not Alternate port "state", but the Alternate port role. The port states and port roles are different from a STP perspective. The Atlernate role was adopted by 802.1w/RSTP, but in the case of 802.1D the right terminology is the non-designated role.