7 Replies Latest reply: Apr 24, 2012 7:57 PM by sarah RSS



      Hi everybody


      I am studying for ccnp switch exam and I have some questions


      Let say we have  a switched network running rstp as shown below:


      sw1 f1/1------------------------------f1/2sw2-----------------------------sw3


      Sw1 is configured with lower priority . We just power our switches up.  Will sw1 and sw2 participate in Proposal/aggreement process before sw2 could put its f1/2 in forwarding state?


      I just watched this video about rstp on ccnp tv.  That video shows the two switches just power on and paticipate in proposal/aggreemnet process before they put their ports in forwarding state.


      I read some post before i posted my question here. One of the post says  in rstp, alternate expects to receive bpdu from designated port; root port expect to receive bpdu from root switch. However the video at CCNP TV shows  in rstp every port transmits bpdu that also includes alternate port. According to the video these bpdus act as keep alive .



      In nut shell, I want to confirm if the switches participate in proposal/aggreement process when they just power on or they only do so when there is toplogy change?


      Do all ports in rstp transmit bpdu( i.e root port,alternate port)?



      thanks and have a great wekend.

        • 1. Re: rstp

          grate paper on this is one by INE 's Petr L. get link or read a quote from



          Yes, all switches will send BPDUs to each other's neighbors. in RSTP's BPDU there are fileds for type of BPDUs,  like TCN BPDU, sync BPDU, ACK BPDU, etc.


          so sw1 and sw 2 agree who is the Root, then sw 2 and sw 3 agree that sw 1 is the root;


          if a sw 3 is up first, it claims to be root, sw 2 agrees before sw 1 comes up online. when sw 1 comes up, sw 1 and 2 exchange BPDUs. sw 2 says sw 3 is the root. sw 1 compares BPDU' it got from sw2 and says no i am the root. sw 2 has to agree and send BPDU with new info back to sw3 and others. sw3 acknowledges that sw 1 is new root and it would send new BPDU to others (if would had other switches connected).


          Unlike Regular STP where only Root switch would send BPDUs, every switch sends its own BPDUs to direct neighbors in RSTP.

          • 2. Re: rstp

            thanks Martin

            • 3. Re: rstp

              Hi everybody.


              Please consider the following scenario:





              rootSW-----Rp SW2-f1/2(D)-----------------f1/1(Rp)SW3


              The network is converged as shown above:


              Let say we add a direct link (fastethernet) between sw3 and rootswitch.


              Following will occur:

              rootswitch sends bpdu with proposal bit set to sw3 on the newly added link


              Sw3 finds the bpdu superior, put its current root port f1/1 in discarding and sends the bpdu with agreement bit set.and put the port connected to newly added link to forwarding state.This is the new root port on sw3.


              root switch receives the bpdu from sw3 with aggreement bit set and put its port connected to newly added link to forwarding state.


              Now focus on sw3.


              What will sw3 do next?  will it sent bpdu with proposal bit set out of its presently discarding former rootport f1/1 to Sw2?



              Or sw3 simply compares the cached bpdu on its f1/1  with its own bpdu and concludes cached bpdu received from sw2 is still superior and put its f1/1 in alternative discarding state without ever sending its own bpdu with proposal bit set to sw2?




              thanks and have a great weekend

              • 4. Re: rstp
                CCNP R&S/CCNA:Wireless,Security

                I tried this out in my lab and as soon as the port with superior BPDU comes online, it becomes the root port.  Former root port becomes an alternate port.

                • 5. Re: rstp

                  What will sw3 do next?


                  sw 3 will send new BPDU to sw 2 informing him about new root port;

                  sw 2 will accept and tries to send one to sw 1 (Root); sw 1 sends back saying I am the Root. sw 2 compares 2 BPDUs now.

                  sw 2 and sw 3 will have to make decision on designated port.  not sure how this happens in details in RSTP but sw 2 should be aware of 2 BPDUs.  in case of same cost to the Root, better BID wins.

                  • 6. Re: rstp
                    CCNP R&S/CCNA:Wireless,Security

                    A TCN BDPU is sent from sw3 to sw2.  sw2 will clear its learned MAC addresses and send a TCN out all designated and root ports accept where TCN was received. sw1 will clear its learned MAC addresses and send TCN out all designated ports accept where TCN was received.  The root advertices TC until max age + forward delay.



                    • 7. Re: rstp

                      Thanks Martin and AaronD


                      You guys pointed out the topology change process which I found quite informative.However , I am just curios if sw3 will send bpdu with proposal bir set to sw2,something( if sw3 indeed does so) that can be viewed by capturing traffic between sw3 and sw2 at that very instant.Unfortunately , I don't have the right gear to verify it. I am using 2691 router with switch module in gns3; i found the module does not support rstp. That is the reason I posted my question hoping somebody with right gear could verify it.


                      thanks and have a wonderful week.