Multiple Instance Spanning-Tree Operation

802.1s Multiple Spanning Tree Protocol (MSTP)

■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 the same instance, all but one of those paths will be blocked for that instance. However, if there are different paths in different instances, all such paths are available for traffic. Separate forwarding paths exist through separate spanning tree instances.

■A port can have different states (forwarding or blocking) for different instances (which represent different forwarding paths).

■MSTP interprets a switch mesh as a single link.

MSTP Compatibility with RSTP or STP

IEEE 802.1s MSTP includes RSTP functionality and is designed to be compatible with both IEEE 802.1D and 802.1w spanning-tree protocols. Using the default configuration values, your switches will interoperate effectively with RSTP and STP devices. MSTP automatically detects when the switch ports are connected to non-MSTP devices in the spanning tree and communicates with those devices using 802.1D or 802.1w STP BPDU packets, as appropriate.

To enable effective interoperation with STP (802.1D) configured devices, however, you may need to adjust the default configuration values. Here are two such examples:

■The rapid state transitions employed by MSTP may result in an increase in the rates of frame duplication and misordering in the switched LAN. To allow the switch to support applications and protocols that may be sensitive to frame duplication and misordering, you can disable rapid transitions by setting the Force Protocol Version parameter to STP-com- patible. The value of this parameter applies to all ports on the switch. See information on force version on page 4-22.

■One of the benefits of MSTP is the implementation of a larger range of port path costs, which accommodates higher network speeds. However, this can create some incompatibility between devices running the older 802.1D STP. You can adjust to this incompatibility by implementing the global spanning-treelegacy-path cost command (see page 4-23). See also the “Note on Path Cost” below.

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