Cisco WS-SUP32-GE-3B Software Configuration Guide - Page 262

Configuring MST Chapter 17 Configuring STP and MST Table 17-7 Default MST Configuration (continued) Feature Maximum-aging time Maximum hop count Default Setting 20 seconds 20 hops MST Configuration Guidelines and Restrictions When configuring MST, follow these guidelines and restrictions: • The 802.1s MST standard allows up to 65 MST instances. You can map an unlimited number of VLANs to an MST instance. • PVST+, rapid PVST+, and MST are supported, but only one version can be active at any time. • VTP does not propagate the MST configuration. You must manually configure the MST configuration (region name, revision number, and VLAN-to-instance mapping) on each switch within the MST region through the command-line interface (CLI) or SNMP. • For load balancing across redundant paths in the network to work, all VLAN-to-instance mapping assignments must match; otherwise, all traffic flows on a single link. • All MST boundary ports must be forwarding for load balancing between a PVST+ and an MST cloud or between a rapid-PVST+ and an MST cloud. For this to occur, the CIST regional root of the MST cloud must be the root of the CST. If the MST cloud consists of multiple MST regions, one of the MST regions must contain the CST root, and all of the other MST regions must have a better path to the root contained within the MST cloud than a path through the PVST+ or rapid-PVST+ cloud. • Partitioning the network into a large number of regions is not recommended. However, if this situation is unavoidable, we recommend that you partition the switched LAN into smaller LANs interconnected by non-Layer 2 devices. • Adding or removing VLANs to an existing MST instance will trigger spanning tree recalculation for that MST instance, and the traffic of all the VLANs for that MST instance will be disrupted. Specifying the MST Region Configuration and Enabling MST For two or more switches to be in the same MST region, they must have the same VLAN-to-instance mapping, the same configuration revision number, and the same MST name. A region can have one member or multiple members with the same MST configuration; each member must be capable of processing RSTP BPDUs. There is no limit to the number of MST regions in a network, but each region can only support up to 65 spanning tree instances. You can assign a VLAN to only one spanning tree instance at a time. To specify the MST region configuration and enable MST, perform this task: Step 1 Step 2 Command Router# configure terminal Router(config)# spanning-tree mst configuration Purpose Enters global configuration mode. Enters MST configuration mode. 17-38 Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY OL-11439-03