802.1s Multiple Spanning Tree Protocol (MSTP)
Within a region, traffic routed between VLANs in separate instances can take only one physical path. To ensure that traffic in all VLANs within a region can travel between regions, all of the boundary ports for each region should belong to all VLANs configured in the region. Otherwise, traffic from some areas within a region could be blocked from moving to other regions.
All MSTP switches (as well as STP and RSTP switches) in a network use BPDUs (Bridge Protocol Data Units) to exchange information from which to build multiple, active topologies in the individual instances within a region and between regions. From this information:
■The MSTP switches in each LAN segment determine a designated bridge and designated port or trunk for the segment.
■The MSTP switches belonging to a particular instance determine the root bridge and root port or trunk for the instance.
■For the IST instance within a region, the MSTP switches linking that region to other regions (or to STP or RSTP switches) determine the IST root bridge and IST root port or trunk for the region. (For any Multiple
■The MSTP switches block redundant links within each LAN segment, across all instances, and between regions, to prevent any traffic loops.
As a result, each individual instance (spanning tree) within a region deter mines its regional root bridge, designated bridges, and designated ports or trunks.
Regions, Legacy STP and RSTP Switches, and the Common Spanning Tree (CST)
The IST instance and any MST instances in a region exist only within that region. Where a link crosses a boundary between regions (or between a region and a legacy STP or RSTP switch), traffic is forwarded or blocked as deter mined by the Common Spanning Tree (CST). The CST ensures that there is only one active path between any two regions, or between a region and a switch running STP and RSTP. (Refer to figure
MSTP Operation with 802.1Q VLANs
As indicated in the preceding sections, within a given MST instance, a single spanning tree is configured for all VLANs included in that instance. This means that if redundant physical links exist in separate VLANs within the same instance, MSTP blocks all but one of those links. However, you can prevent the bandwidth loss caused by blocked redundant links for different VLANs in