IEEE 802.1s Multiple STP (MSTP) helps create multiple loop-free active topologies on a single physical topology.
MSTP uses RSTP to group VLANs into separate spanning-tree instance. Each instance has its own spanning-tree topology independent of other spanning tree instances, which allows multiple forwarding paths, permits load balancing, and facilitates the movement of data traffic. A failure in one instance does not affect other instances. By enabling the MSTP, you are able to more effectively utilize the physical resources present in the network and achieve better load balancing of VLAN traffic.
The MSTP evolved as a compromise between the two extremes of the RSTP and R-PVST+, it was standardized as IEEE 802.1s and later incorporated into the IEEE 802.1Q-2003 standard. The MSTP configures a meshed topology into a loop-free, tree-like topology. When the link on a bridge port goes up, an MSTP calculation occurs on that port. The result of the calculation is the transition of the port into either a forwarding or blocking state. The result depends on the position of the port in the network and the MSTP parameters. All the data frames are forwarded over the spanning tree topology calculated by the protocol.
Note
Multiple switches must be configured consistently with the same MSTP configuration to participate in multiple spanning tree instances. A group of interconnected switches that have the same MSTP configuration is called an MSTP region.
MSTP is backward compatible with the STP and the RSTP.
The single Spanning Tree instance used by the Extreme device, and other vendor devices to interoperate with MSTP bridges. This spanning tree instance stretches across the entire network domain (including PVST, PVST+ and MSTP regions). It is associated with VLAN 1 on the Extreme device.
An MSTP bridge must handle at least these two instances: one IST and one or more MSTIs (Multiple Spanning Tree Instances). Within each MST region, the MSTP maintains multiple spanning-tree instances. Instance 0 is a special instance known as IST, which extends CST inside the MST region. IST always exists if the device runs MSTP. Besides IST, this implementation supports up to 31 MSTIs.
The single spanning tree calculated by STP (including PVST+) and RSTP (including R-PVST+) and the logical continuation of that connectivity through MSTP bridges and regions, calculated by MSTP to ensure that all LANs in the bridged LAN are simply and fully connected
One of a number of spanning trees calculated by the MSTP within an MST Region, to provide a simply and fully connected active topology for frames classified as belonging to a VLAN that is mapped to the MSTI by the MST configuration table used by the MST bridges of that MST region.
The Extreme implementation supports up to 32 spanning tree instances in an MSTP enabled bridge that can support up to 32 different Layer 2 topologies. The spanning tree algorithm used by the MSTP is the RSTP, which provides quick convergence.
By default all configured VLANs including the default VLAN are assigned to and derive port states from CIST until explicitly assigned to MSTIs.
MST regions are clusters of bridges that run multiple instances of the MSTP protocol. Multiple bridges detect that they are in the same region by exchanging their configuration (instance to VLAN mapping), name, and revision-level. Therefore, if you need to have two bridges in the same region, the two bridges must have identical configurations, names, and revision-levels. Also, one or more VLANs can be mapped to one MST instance (IST or MSTI) but a VLAN cannot be mapped to multiple MSTP instances
MSTP introduces a hierarchical way of managing device domains using regions. Devices that share common MSTP configuration attributes belong to a region. The MSTP configuration determines the MSTP region where each device resides. The common MSTP configuration attributes are as follows:
Region boundaries are determined by the above attributes. An MSTI is an RSTP instance that operates inside an MSTP region and determines the active topology for the set of VLANs mapping to that instance. Every region has a CIST that forms a single spanning tree instance which includes all the devices in the region. The difference between the CIST instance and the MSTP instance is that the CIST instance operates across the MSTP region and forms a loop-free topology across regions, while the MSTP instance operates only within a region. The CIST instance can operate using the RSTP only if all the devices across the regions support the RSTP. However, if any of the devices operate using the STP, the CIST instance reverts to the STP.
Each region is viewed logically as a single STP or a single RSTP bridge to other regions.
Note
Extreme supports 32 MSTP instances and one MSTP region.For more information about spanning trees, see the introductory sections in the Spanning Tree Protocol chapter.