Multiple Instance Spanning-Tree Operation

802.1s Multiple Spanning Tree Protocol (MSTP)

Common and Internal Spanning Tree (CIST): The CIST identifies the regions in a network and administers the CIST root bridge for the network, the root bridge for each region, and the root bridge for each spanning-tree instance in each region.

Common Spanning Tree (CST): The CST administers the connectivity among the MST regions, STP LANs, and RSTP LANs in a bridged network.

MST Region: An MST region comprises the VLANs configured on physically connected MSTP switches. All switches in a given region must be configured with the same VLANs, the same Multiple Spanning Tree Instances (MSTIs), and the same MST configuration identifiers.

Internal Spanning Tree (IST): The IST administers the topology within a given MST region. When you configure a switch for MSTP operation, the switch automatically includes all of the static VLANs configured on the switch in a single, active spanning tree topology (instance) within the IST. This is termed the “IST instance”. Any VLANs you subsequently configure on the switch are added to this IST instance. To create separate forwarding paths within a region, group specific VLANs into different Multiple Spanning Tree Instances (MSTIs). (Refer to “Multiple Spanning Tree Instance (MSTI)”, below.)

Types of Multiple Spanning Tree Instances: A multiple spanning tree network comprises separate spanning-tree instances existing in an MST region. (There can be multiple regions in a network.) Each instance defines a single forwarding topology for an exclusive set of VLANs. By contrast, an STP or RSTP network has only one spanning tree instance for the entire network, and includes all VLANs in the network. (An STP or RSTP network operates as a single-instance network.) A region can include two types of STP instances:

■Internal Spanning-Tree Instance (IST Instance): This is the default spanning tree instance in any MST region. It provides the root switch for the region and comprises all VLANs configured on the switches in the region that are not specifically assigned to Multiple Spanning Tree Instances (MSTIs, described below).

Within a region, the IST instance provides a loop-free forwarding path for all VLANs associated with it. VLANs that are not associated with an MSTI are, by default, associated with the IST instance. Note that the switch automatically places dynamic VLANs (resulting from GVRP operation) in the IST instance. Dynamic VLANs cannot exist in an MSTI (described below).

■Multiple Spanning Tree Instance (MSTI): This type of configurable spanning tree instance comprises all static VLANs you specifically assign to it, and must include at least one VLAN. The VLAN(s) you assign to an

4-8