Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 
New Member



If I connect switch that runs MST spanning-tree with other Catalyst that runs PVST spanning tree, should I expect problems (they are Catalyst 6500 and 35600 interconnected with trunk)? I heard that maybe some VLANs won't be propagated and that topology won't be stable. Does anyone have experience with that?



New Member


When a cisco MST bridge interacts with a cisco PVST bridge on a trunk port, it replicates the IST BPDUs as many times as needed on that trunk to simulate the PVST (i.e. if 100 VLANs are defined, the MST bridge replicates the IST BPDU 100 times). Now what you have to remember is that an MST bridge can only interact with another region or a non-MST bridge via the IST/CST. The PVST simulation fails (and puts the boundary port in root-inconsistent) if the MST bridge is the

root for the IST while simultaneously receiving superior BDPUs on different VLANs from the PVST brige.

We have seen that an MST region interacts with a PVST bridge via the IST/CST. However, a PVST+ bridge runs one spanning-tree per VLAN. On a trunk port where there are N VLANs, this translates to either sending or receiving one BPDU on each VLAN every 2 seconds. It is important to know at this point that Cisco switches running the PVST+ algorithm always

run the CST on VLAN 1 on trunk ports, whether VLAN 1 is the native VLAN or not. That is, BPDUs for the CST always contain VLAN 1 information and are sent untagged on the native VLAN to the IEEE address 01-80-C2-00-00-00. So what happens if you interconnect an MST bridge with a PVST+ bridge via a trunk port? The MST bridge enters a PVST+ simulation mode

where it is capable of replicating a PVST+ BPDU N times and sending them out to the PVST+ bridge. However, there are certain rules that need to be followed for the PVST+ Simulation to function properly:

4. if the MST bridge is the root, then it must be the root for all VLANs

5. if the PVST+ bridge is the root, then it must be root for all VLANs (including the CST)

6. the simulation will fail if the MST bridge is the root for the CST while the PVST+ bridge is the root for other VLANs  a failed simulation puts the boundary port in root inconsistent mode. The MST bridge will retry the simulation every 30 seconds.

CreatePlease to create content