Cisco switches running MST connected to non-MST domains run PVST simulation automatically on a per-port basis when PVST BPDUs are received. In such cases, the following rules must be followed for better convergence:
- If the root bridge for CIST is within a non-MST region where MST is not running, the spanning-tree priority of VLANs 2 and above within that domain must be lesser than that of VLAN 1.
- If the root bridge for CIST is within an MST region, VLANs 2 and above that are defined in the non-MST domains must have their spanning-tree priorities greater than that of the CIST root.
Test steps
- Create two tagged streams of VLAN ID 10 and 20 with source port 3, and destination ports 1 and 2.
- Shut down e1/1/9 on S4128-SW1 switch and measure traffic loss and duration of traffic loss.
- Recover e1/1/9 on S4128-SW1 and measure traffic loss and duration of traffic loss.
Results
VLAN 1, the default VLAN in OS10, converges with mst0 in Cisco. All other VLAN instances in RPVST+ and other MSTI in the MSTP do not converge.