6 Replies Latest reply: Apr 5, 2012 1:38 PM by Ian RSS



      Hi all,


      I've gone blank here.  Why would we use STP different VLAN's on a switch and not one STP with one root bridge?



        • 1. Re: STP
          Elvin Arias

          Because with different logical STP topologies you can accomplish traffic engineering from a L2 perspective. Normally on older STPs load balancing options were not available, since there is only one root bridge on the network that is the root for ALL the traffic, but in the case of, for example, PVST+ this can easily accomplished by tunning different root bridges on the L2 network.



          • 2. Re: STP

            I understand that you can have different root bridges for different VLAN's,  I just can't think of a scenario why, my head has gone blank.

            • 3. Re: STP

              Once a network gets to be one very large broadcast domain on the LAN, the performance starts to diminish due to the large amount of broadcast traffic. 

              • 4. Re: STP
                Elvin Arias

                Let's say you have three switches interconnected like a "ring", so redundancy is accomplished, but let's named the switches as SW1, SW2, and SW3, and let's assume there are two VLANs (VLAN 2, and VLAN 3) on the network. If you make, let's say, SW2 the root bridge for all the VLANs (VLAN 2, and VLAN 3 in this case) you will waste resources, since you will clearly have to elect just ONE trunk port in order to send traffic through the network, buts let's distritbute the root bridge charge, and designate SW3 as the root for one of the VLANs, let's say VLAN 3. In this case you (as SW1) will be sending VLAN 2 traffic directly to SW2, and for SW3 there is a direct distribution of traffic for VLAN 3 too.


                In this way you will not have an entire trunk blocked (in the alternate role) for ALL your VLANs.



                • 5. Re: STP

                  Elvin, I think I understand, I will build this in packet trace I think, unless you have an example already for me to load.  If you have a root for one VLAN on one switch and the root for another VLAN on another switch wont all the ports change their blocking ports etc?

                  • 6. Re: STP

                    Be carefull with that approach (load balancing vlans with pvst).  It's good to use, because you have traffic on all links and can verify they work bidirectionally.  This can however make people think they have more bandwidth if they don't calc the usage in a failure scenario in the real-world.


                    I realize this won't be an issue in packet trace, but something to keep in mind in the production environment.