Cisco 7609-S Software Configuration Guide - Page 289

Chapter 20 Configuring STP and Prestandard IEEE 802.1s MST Understanding How Prestandard IEEE 802.1s MST Works • MST Instances, page 20-18 • MST Configuration Parameters, page 20-18 • MST Regions, page 20-19 • Message Age and Hop Count, page 20-20 • Default STP Configuration, page 20-21 IEEE 802.1s MST Overview MST in this release is based on the draft version of the IEEE standard. 802.1s for MST is an amendment to 802.1Q. MST extends the IEEE 802.1w rapid spanning tree (RST) algorithm to multiple spanning trees. This extension provides both rapid convergence and load balancing in a VLAN environment. MST converges faster than PVST+. MST is backward compatible with 802.1D STP, 802.1w (rapid spanning tree protocol [RSTP]), and the Cisco PVST+ architecture. MST allows you to build multiple spanning trees over trunks. You can group and associate VLANs to spanning tree instances. Each instance can have a topology independent of other spanning tree instances. This new architecture provides multiple forwarding paths for data traffic and enables load balancing. Network fault tolerance is improved because a failure in one instance (forwarding path) does not affect other instances (forwarding paths). In large networks, you can more easily administer the network and use redundant paths by locating different VLAN and spanning tree instance assignments in different parts of the network. A spanning tree instance can exist only on bridges that have compatible VLAN instance assignments. You must configure a set of bridges with the same MST configuration information, which allows them to participate in a specific set of spanning tree instances. Interconnected bridges that have the same MST configuration are referred to as an MST region. MST uses the modified RSTP version called the Multiple Spanning Tree Protocol (MSTP). The MST feature has these characteristics: • MST runs a variant of spanning tree called internal spanning tree (IST). IST augments the common spanning tree (CST) information with internal information about the MST region. The MST region appears as a single bridge to adjacent single spanning tree (SST) and MST regions. • A bridge running MST provides interoperability with single spanning tree bridges as follows: - MST bridges run IST, which augments the common spanning tree (CST) information with internal information about the MST region. - IST connects all the MST bridges in the region and appears as a subtree in the CST that includes the whole bridged domain. The MST region appears as a virtual bridge to adjacent SST bridges and MST regions. - The common and internal spanning tree (CIST) is the collection of ISTs in each MST region, the CST that interconnects the MST regions, and the SST bridges. CIST is the same as an IST inside an MST region and the same as CST outside an MST region. The STP, RSTP, and MSTP together elect a single bridge as the root of the CIST. OL-4266-08 Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX 20-15