21-16
Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-21521-01
Chapter 21 Configuring MSTP
Configuring MSTP Features
• 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 IST master of the MST
cloud should also 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. You might have to manually configure the switches in the clouds.
• 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 routers or non-Layer 2 devices.
• For configuration guidelines about UplinkFast, BackboneFast, and cross-stack UplinkFast, see the
“Optional Spanning-Tree Configuration Guidelines” section on page 22-12.
• When the switch is in MST mode, it uses the long path-cost calculation method (32 bits) to compute
the path cost values. With the long path-cost calculation method, these path cost values are
supported:
Specifying the MST Region Configuration and Enabling MSTP
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 name.
A region can have one member or multiple members with the same MST configuration; each member
mu
st 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.
Beginning in privileged EXEC mode, follow these steps to specify the MST region configuration and
enabl
e MSTP. This procedure is required.
Speed Path Cost Value
10 Mb/s 2,000,000
100 Mb/s 200,000
1 Gb/s 20,000
10 Gb/s 2,000
100 Gb/s 200
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
spanning-tree mst configuration Enter MST configuration mode.
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