9 Replies Latest reply: Sep 5, 2011 6:44 PM by Martin RSS

    Per Vlan (R)STP


      Hello everybody,


      I am reading CCNA Official Exam Certification Guide and I have encountered some problems. I cannot really feel the “(R)STP instance for every VLAN” thing. IMHO the example in the book I mentioned is too easy (there are only 2 switches with the same VLAN database).


      Let’s think about the topology given below:


      Everywhere, there are trunk links between switches. Every switch has one VLAN1 and all interfaces in it, except:

      Switch0 – additionl VLAN10 and access interface in this VLAN to PC1

      Switch5 – additional VLAN10 and access interface in this VLAN to PC0

      Switch5 – additional VLAN333 with no interfaces assigned

      Switch1 - additional VLAN33 with no interfaces assigned


      I have checked sh span vlan 10 on Switch0 and Switch5. Output shows that both switches are Root bridges… I thought STP for VLAN10 should be sth like this:


      For VLAN333 sth like this:

      And for VLAN1 for example sth like this:

      What have I misunderstood? Please, take your time and help me.

        • 1. Re: Per Vlan (R)STP

          You can upload your PT file here; Attach to this post using Advanced Editor;

          Output shows that both switches are Root bridges

          One root per Vlan, as you cannot have 2 root bridges on 1 same vlan;

          Make sure all switches have the same vlans; then check which one is Root for which vlan;


          usually you will have to make changes manually, changing bridge priority for vlans;

          • 2. Re: Per Vlan (R)STP

            Martin, thanks for your reply.


            > Make sure all switches have the same vlans

            I do not want all switches to have the same vlan database - then it'd be easy I guess. I want to get to know how (R)STP works in this particular situation


            > you cannot have 2 root bridges on 1 same vlan

            But they are. PKT file in the attachment.


            STP for VLAN1 is correct. You can check it in Simulation mode - Switch4 is whilst Switch5 is But this is the only VLAN where the STP works properly. I cannot understand behaviour of STP for VLAN10 and VLAN333.

            • 3. Re: Per Vlan (R)STP

              One more thing


              Why in PT can I ping PC0-PC1 whilst there is no VLAN10 on Switch2?


              I physically connect three switches (SW1---SW2---SW3), with VLAN10 in edges switches only (SW1, SW3) and there is no connectivity!


              Now I am confused even more...

              • 4. Re: Per Vlan (R)STP

                I could not open your file, but here is mine




                open it and take a look at it;  see if we talk about the same things;


                all switches have same vlan database;

                example 1 is with 1 sw as root for all vlans;

                example 2 is with mixed root switches per vlans

                • 5. Re: Per Vlan (R)STP

                  Yes, Martin I understand this example.


                  Mine is different because VLAN database is not the same on all switches. Is it required for STP that all switches has the same VLANS? If is so, why PC0 can ping PC1 in my example. In route of this ping, there are Swich0, 2, 5 and there is no VLAN10 in Switch2.


                  P.S I use PT 5.3.2. Martin probably you have older one and that is why you cannot open my file.

                  • 6. Re: Per Vlan (R)STP


                    this shouldn't work but in PT what's stranger is that switch 2 hasn't got the mac address of the source pinging but switch 5 has it from its link to switch2.


                    I really think PT is screwing things up but in real life this should not work.



                    • 7. Re: Per Vlan (R)STP
                      Brian McGahan - 4 x CCIE, CCDE

                      > you cannot have 2 root bridges on 1 same vlan

                      But they are. PKT file in the attachment.



                      You can, but it indicates an error condition.  If two bridges that are supposed to be sharing the same VLAN both claim themselves to be the root, it means that the STP BPDUs are not being exchanged between those switches for that VLAN.


                      For example if I claim myself to be the root for VLAN 10, and you do the same thing, it means that VLAN 10 communication is broken between us.  A common case that this occurs is that a trunk link has formed to fail somewhere in the topology, or the VLAN in question is being filtered off of a trunk link.

                      • 8. Re: Per Vlan (R)STP

                        Ok thank you all. So, I assume there should be the same vlan database on all switches in topology (in all switches between hosts in the same VLAN to be precised). Fine but no really - please correct me if I made mistake:


                        It means that if I have 100 switches and only 2 with access interfaces in VLAN 10 I have to add VLAN 10 on all switches beetween those two (98 in extrame case)? OK, you probably will say about VTP in Cisco switches. Fine, thanks for VTP there will be VLAN10 in all switches but without interfaces assigned (but it doesn't matter, it matters that VLAN is in the database to switch VLAN 10 in trunk link, I guess). OK, but what shall I do when 50 of my 100 switches are from different vendor? That means that I have to manually create VLAN 10 in 50 switches in order to trunk VLAN10 across the entire network ?!

                        • 9. Re: Per Vlan (R)STP
                          Martin probably you have older one and that is why you cannot open my file.

                          yes, another PC, another PT version, hehehehe


                          be carefull with PT running some complex STP things; it does not work very well;


                          There are management tools for Vlan management for Cisco and no-Cisco;

                          f.e Ciscoworks or Solarwinds;