Spanning-Tree Operation

802.1s Multiple Spanning Tree Protocol (MSTP)

Operating Rules

■All switches in a region must be configured with the same set of VLANs, as well as the same MST configuration name and MST configuration number.

■Within a region, a VLAN can be allocated to either a single MSTI or to the region’s IST instance.

■All switches in a region must have the same VID-to-MST instance and VID- to-IST instance assignments.

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There is one root MST switch per configured MST instance.

Within any region, the root switch for the IST instance is also the root switch for the region. Because boundary ports provide the VLAN connec­ tivity between regions, all boundary ports on a region's root switch should be configured as members of all static VLANs defined in the region.

■There is one root switch for the Common and Internal Spanning Tree (CIST). Note that the per-port hello-timeparameter assignments on the CIST root switch propagate to the ports on downstream switches in the network and override the hello-timeconfigured on the downstream switch ports.

■Where multiple MST regions exist in a network, there is only one active, physical communication path between any two regions, or between an MST region and an STP or RSTP switch. MSTP blocks any other physical paths as long as the currently active path remains in service.

■Within a network, an MST region appears as a virtual RSTP bridge to other spanning tree entities (other MST regions, and any switches running 802.1D or 802.1w spanning-tree protocols).

■Within an MSTI, there is one spanning tree (one physical, communication path) between any two nodes. That is, within an MSTI, there is one instance of spanning tree, regardless of how many VLANs belong to the MSTI. Within an IST instance, there is also one spanning tree across all VLANs belonging to the IST instance.

■An MSTI comprises a unique set of VLANs and forms a single spanning- tree instance within the region to which it belongs.

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Communication between MST regions uses a single spanning tree.

If a port on a switch configured for MSTP receives a legacy (STP/802.1D or RSTP/802.1w) BPDU, it automatically operates as a legacy port. In this case, the MSTP switch interoperates with the connected STP or RSTP switch as a separate MST region.

■Within an MST region, there is one logical forwarding topology per instance, and each instance comprises a unique set of VLANs. Where multiple paths exist between a pair of nodes using VLANs belonging to

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