Cisco 2950 Software Configuration Guide - Page 320
Understanding MSTP, Multiple Spanning-Tree Regions, IST, and CST
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Understanding MSTP Chapter 15 Configuring MSTP Understanding MSTP MSTP, which uses RSTP for rapid convergence, enables VLANs to be grouped 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. These sections describe how the MSTP works: • Multiple Spanning-Tree Regions, page 15-2 • IST, CIST, and CST, page 15-2 • Hop Count, page 15-4 • Boundary Ports, page 15-5 • Interoperability with 802.1D STP, page 15-5 For configuration information, see the "Configuring MSTP Features" section on page 15-11. Multiple Spanning-Tree Regions For switches to participate in multiple spanning-tree (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 15-1 on page 15-4. The MST configuration determines 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. You configure the switch for a region by using the spanning-tree mst configuration global configuration command, after which the switch enters the MST configuration mode. From this mode, you can map VLANs to an MST instance by using the instance MST configuration command, specify the region name by using the name MST configuration command, and set the revision number by using the revision MST configuration command. A region can have one member or multiple members with the same MST configuration; each member must be capable of processing RSTP bridge protocol data units (BPDUs). There is no limit to the number of MST regions in a network, but each region can support up to 16 spanning-tree instances. You can assign a VLAN to only one spanning-tree instance at a time. IST, CIST, and CST Unlike PVST+ and rapid PVST+ in which all the spanning-tree instances are independent, the MSTP establishes and maintains two types of spanning trees: • An internal spanning tree (IST), which is the spanning tree that runs in an MST region. Within each MST region, the MSTP maintains multiple spanning-tree instances. Instance 0 is a special instance for a region, known as the internal spanning tree (IST). All other MST instances are numbered from 1 to 15. The IST is the only spanning-tree instance that sends and receives BPDUs; all of the other spanning-tree instance information is contained in M-records, which are encapsulated within MSTP BPDUs. Because the MSTP BPDU carries information for all instances, the number of BPDUs that need to be processed by a switch to support multiple spanning-tree instances is significantly reduced. 15-2 Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide 78-11380-10