Multiple Spanning Tree Regions
Using MSTP, the entire network runs a common instance of RSTP. Within that common instance, one or more VLANs can be individually configured into distinct regions. The entire network runs the common spanning tree instance (CST) and the regions run a local instance. The local instance is known as Internal Spanning Tree (IST). The CST treats each instance of IST as a single bridge. Consequently, ports are blocked to prevent loops that might occur within an IST and also throughout the CST. With the exception of the provisions for multiple instances, MSTP operates exactly like RSTP.
For example, in MSTP Configured Network a network is configured with two regions: Region 1 and Region 2. The entire network is running an instance of CST. Each of the regions is running an instance of IST. In addition, this network contains Switch 1 running MSTP that is not configured in a region and consequently is running in the CIST instance. In this configuration, the regions are each regarded as a single bridge to the rest of the network, as is Switch 1. The CST prevents loops from occurring across the network. Consequently, a port is blocked at port 1/1/2 of switch 4.
Additionally, loops must be prevented in each of the IST instances. Within the IST Region 1, a port is blocked at port 1/1/2 of switch 4 to prevent a loop in that region. Within Region 2, a port is blocked at port 1/3/2 of switch 3 to prevent a loop in that region.
The following definitions describe the STP instances that define an MSTP configuration.
Common Spanning (CST): CST is defined in 802.1q and assumes one spanning-tree instance for the entire bridged network regardless of the number of VLANs. In MSTP, an MSTP region appears as a virtual bridge that runs CST.
Internal Spanning Tree (IST): IST is a new terminology introduced in 802.1s. 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 switch runs MSTP. Besides IST, this implementation supports up to 15 MSTIs, numbered from 1 to 4094.
Common and Internal Spanning Trees (CIST): CIST is a collection of the ISTs in each MST region and the CST that interconnects the MST regions and single spanning trees.
Multiple Spanning Tree Instance (MSTI): The MSTI is identified by an MST identifier (MSTid) value between 1 and 4094.
MSTP Region: These 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 MSTP instance (IST or MSTI) but a VLAN cannot be mapped to multiple MSTP instances.