Cisco WS-SUP32-GE-3B Software Configuration Guide - Page 242

Understanding MST Chapter 17 Configuring STP and MST • IST, CIST, and CST, page 17-19 • Hop Count, page 17-22 • Boundary Ports, page 17-22 • Standard-Compliant MST Implementation, page 17-23 • Interoperability with IEEE 802.1D-1998 STP, page 17-25 MST Overview MST maps multiple VLANs into a spanning tree instance, with each instance having a spanning tree topology independent of other spanning tree instances. This architecture provides multiple forwarding paths for data traffic, enables load balancing, and reduces the number of spanning tree instances required to support a large number of VLANs. MST improves the fault tolerance of the network because a failure in one instance (forwarding path) does not affect other instances (forwarding paths). The most common initial deployment of MST is in the backbone and distribution layers of a Layer 2 switched network. This deployment provides the kind of highly available network that is required in a service-provider environment. MST provides rapid spanning tree convergence through explicit handshaking, which eliminates the 802.1D forwarding delay and quickly transitions root bridge ports and designated ports to the forwarding state. MST improves spanning tree operation and maintains backward compatibility with these STP versions: • Original 802.1D spanning tree • Existing Cisco-proprietary Multiple Instance STP (MISTP) • Existing Cisco per-VLAN spanning tree plus (PVST+) • Rapid per-VLAN spanning tree plus (rapid PVST+) For information about PVST+ and rapid PVST+, see Chapter 17, "Configuring STP and MST." For information about other spanning tree features such as Port Fast, UplinkFast, root guard, and so forth, see Chapter 18, "Configuring Optional STP Features." Note • IEEE 802.1w defined the Rapid Spanning Tree Protocol (RSTP) and was incorporated into IEEE 802.1D. • IEEE 802.1s defined MST and was incorporated into IEEE 802.1Q. MST Regions For switches to participate in MST instances, you must consistently configure the switches with the same MST configuration information. A collection of interconnected switches that have the same MST configuration comprises an MST region as shown in Figure 17-10 on page 17-21. The MST configuration controls to which MST region each switch belongs. The configuration includes the name of the region, the revision number, and the MST VLAN-to-instance assignment map. 17-18 Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY OL-11439-03