Multiple Instance Spanning-Tree Operation

802.1s Multiple Spanning Tree Protocol (MSTP)

Terminology

Bridge: See “MSTP Bridge”.

Common and Internal Spanning Tree (CIST): Comprises all LANs, STP, and RSTP bridges and MSTP regions in a network. The CIST automatically determines the MST regions in a network and defines the root bridge (switch) and designated port for each region. The CIST includes the Common Spanning Tree (CST), the Internal Spanning Tree (IST) within each region, and any multiple spanning-tree instances (MSTIs) in a region.

Common Spanning Tree (CST): Refers to the single forwarding path the switch calculates for STP (802.1D) and RSTP (802.1w) topologies, and for inter-regional paths in MSTP (802.1s) topologies. Note that all three types of spanning tree can interoperate in the same network. Also, the MSTP switch interprets a device running 802.1D STP or 802.1w RSTP as a separate region. (Refer to figure 5-2 on page 7.)

Internal Spanning Tree (IST): Comprises all VLANs within a region that are not assigned to a multiple spanning-tree instance configured within the region. All MST switches in a region should belong to the IST. In a given region “X”, the IST root switch is the regional root switch and provides information on region “X” to other regions.

MSTP (Multiple Spanning Tree Protocol): A network supporting MSTP allows multiple spanning tree instances within configured regions, and a single spanning tree among regions, STP bridges, and RSTP bridges.

MSTP BPDU (MSTP Bridge Protocol Data Unit): These BPDUs carry region-specific information, such as the region identifier (region name and revision number). If a switch receives an MSTP BPDU with a region identifier that differs from its own, then the port on which that BPDU was received is on the boundary of the region in which the switch resides.

MSTP Bridge: In this manual, an MSTP bridge is a 2510G Switch (or another 802.1s-compatible device) configured for MSTP operation.

MST Region: An MST region forms a multiple spanning tree domain and is a component of a single spanning-tree domain within a network. For switches internal to the MST region:

■All switches have identical MST configuration identifiers (region name and revision number).

■All switches have identical VLAN assignments to the region’s IST and (optional) MST instances.

■One switch functions as the designated bridge (IST root) for the region.

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