MSTP operating rules

  • All switches in a region must be configured with the same set of VLANs, the same MST configuration name and MST configuration number.

  • Within a region, a VLAN can be allocated to either a single MSTI or to the region's IST instance.

  • All switches in a region must have the same VID-to-MST instance assignment.

  • There is one root MST switch per configured MST instance.

  • Because boundary ports provide the VLAN connectivity between regions, all boundary ports on a region's root switch should be configured as members of all static VLANs defined in the region.

  • There is one root switch for the Common and Internal Spanning Tree (CIST). At any given time, all switches in the network will use the per-port hello-time parameter assignments configured on the CIST root switch.

  • Where multiple MST regions exist in a network, there is only one active, physical communication path between any two regions, or between an MST region and an STP or RSTP switch. MSTP blocks any other physical paths as long as the currently active path remains in service.

  • Within a network, an MST region appears as a virtual RSTP bridge to other spanning tree entities (other MST regions, and any switches running 802.1D or 802.1w spanning tree protocols).

  • Within an MSTI, there is one physical communication path between any two nodes, regardless of how many VLANs belong to the MSTI. Within an IST instance, there is also one spanning tree across all VLANs belonging to the IST instance.

  • An MSTI comprises a unique set of VLANs and forms a single spanning tree instance within the region to which it belongs.

  • A dynamic VLAN learned by GVRP will always be placed in the IST instance and cannot be moved to any configured MST instance.Starting in software release 13.X.X, dynamically learned GVRP VLANs can be mapped to MSTIs and support MSTP load balancing.

  • In software release 13.X.X and later, you can preconfigure static and dynamic VLAN ID-to-MSTI mappings before the VLAN is created on the switch. Later, when the static VLAN ID is configured or a dynamic GVRP VLAN is learned, the VLAN is automatically associated with the preconfigured MSTI.

  • Communication between MST regions uses a single spanning tree.

  • If a port on a switch configured for MSTP receives a legacy (STP/802.1D or RSTP/802.1w) BPDU, it automatically operates as a legacy port. In this case, the MSTP switch interoperates with the connected STP or RSTP switch as a separate MST region.

  • Within an MST region, there is one logical forwarding topology per instance, and each instance comprises a unique set of VLANs. Where multiple paths exist between a pair of nodes using VLANs belonging to the same instance, all but one of those paths will be blocked for that instance. However, if there are different paths in different instances, all such paths are available for traffic. Separate forwarding paths exist through separate spanning tree instances.

  • A port can have different states (forwarding or blocking) for different instances (which represent different forwarding paths).

  • MSTP interprets a switch mesh as a single link.