VSX supports the Multiple Spanning Tree Protocol (MSTP). Both VSX switches appear as a single common Spanning Tree Bridge ID to MSTP partner devices upstream and downstream that participate to the same Spanning Tree domain. MSTP can be enabled on VSX switches and any nonrouting ports both VSX lags and non-VSX lags can participate in MSTP topology and takes decision to avoid any loops.

MSTP on VSX uses the same bridge ID with the same MAC address on VSX LAGs and non-VSX LAGs, orphan ports. This MAC address is referred to as a common Bridge ID which consists of Spanning Tree priority and the switch MAC Address. The STP port state is the same for VSX LAG ports in VSX peer switches.

The Spanning Tree protocol runs independently on VSX nodes, which conforms to the dual-control plane VSX architecture. The primary VSX node is responsible to run the protocol for the VSX LAGs. In the normal state, the primary is "Operational Primary" and the secondary is "Operational Secondary". If a primary VSX node failure occurs, the secondary VSX node becomes the MSTP Operational Primary. When the Primary VSX node goes back up, it takes back ownership of the MSTP Operational Primary role.

On VSX LAG ports, MSTP runs only from the Operational Primary. The Operational Secondary holds precomputed MSTP information for ready-state switch over thanks to MSTP states synchronization done by Operational Primary to Operational Secondary for links member of the VSX LAG. That happens as a part of the initial sync (LACP, MAC, ARP, MSTP). During switch-over, BPDU to downstream or upstream are sent by the new Operational Primary within the default of 6 seconds of the Spanning Tree BPDU failure detection timer: 3x hello-timer (2s per default).

ISL is always part of MSTP, nonblocking and it sends and receives BPDUs.


The VSX synchronization does not support MSTP. The MSTP configuration on both VSX nodes must be identical.

Multiple MSTP regions are not supported. Multiple instances are supported though (default + 64).


Do not use system-mac-1, system-mac, or system-mac+1 on any node of the network. For the internal Spanning Tree protocol between VSX nodes, the Bridge_ID of the primary and secondary VSX nodes are derived from (-1, +1) from VSX system-mac functionality.


Path cost is not allowed to be configured on an ISL port.

Sample MSTP on VSX configurationSample MST on VSX configuration