关于pvst或者pvst+技术的讨论核心是：为了每个vlan生成一个spanning-tree，是否有必要。每2秒钟一个bpdu包，会不会对交换机形成巨大的压力。
解决方案是采用新的mst，其实就是建立2颗spanning-tree，每组里面放1半的vlan，然后建立桥优先级互相备份。
配置参考：
Configuring MST
This section provides the commands you will need in order to set up the basic configuration of MST on your Catalyst 4000, 6000, or 6500 series switch. Commands and explanations for tuning MST follow the basic configuration section.
Basic Configuration
Follow these steps:
Enable MST on the switch.
Use the set spantree mode mst command to set the spanning tree mode on the switch to MST.
Note: To disable MST, another spanning tree protocol, such as PVST+, must be configured.
Define the VLAN-to-instance mappings.
Use the set spantree MST instance vlan vlans command to map VLANs to an instance. For example, you would enter the command set spantree MST 10 vlan 1-10,20 to put VLANs 1 to 10 and 20 into instance 10. By default, all VLANs are mapped to instance 0.
Note: Mapping a VLAN to an instance does not take affect until the configuration is committed.
Define the MST configuration name and revision.
Use the set spantree MST configuration name name and the set spantree MST configuration revision revision number commands to set the configuration and the revision.
Note: Instances 1 to 15 only operate within the MST region. On the boundary of the MST region, MST copies the port state from the IST, which communicates with the other spanning tree protocols such as PVST+, CST, and other MST regions to form a loop-free topology. MST-enabled switches only form an MST region if they have a matching VLAN-to-IST mapping, MST configuration name, and MST revision. If any of these three fails, the port will be flagged as a boundary port.
Commit the MST configuration to apply it on the switch.
Use the set spantree MST config commit command to commit the MST configuration.
Note: If you find that you need to discard all edits made since the last commit, you can use the set spantree MST rollback command to undo all edits. If you need to clear changes to the MST configuration made by someone else using another session, use the set spantree MST rollback force command.
Tuning MST
Use the following commands to tune MST on your Catalyst switch.
Setting Priority Per MST Instance
Because every instance in MST runs independently of the other instances on the switch, every instance can have a different priority on the switch and possibly a different root in the MST region.
To set the priority of a spantree in an instance, use the set spantree priority priority MST instance command.
Setting Path Costs on a Specific Port
For every port, MST can use either the standard cost for the port or it can assign a different cost using the set spantree portinstancecost mod/port cost cost MST instance command. You can use this command to specify the alternative path cost for the link being used and to specify which instances will use this alternative path cost.
Setting Port Priority on a Specific Port
For every port, MST can use either the standard priority for the port or it can assign a different priority with the set spantree portinstancepriority mod/port priority MST instance command. You can use this command to specify an alternative priority for a port and to specify which instance will use the alternative priority.
Setting the Link Type on a Port
Using the set spantree MST link-type mod/port link-type command, you can configure the link type in any of the following three ways.
Auto - The switch will autodetect the link type for MST.
Point-to-point - The link is a point-to-point link to another device. For example, you could have a 10 Gigabit link to another Catalyst switch.
Shared - The link is a shared segment and can contain more than one device. An example of such a link would be a 10 Mb hub.
Verifying MST Operation
This section provides commands you can use to confirm your configuration is working properly. Examples of output from these commands are explained in the Troubleshooting section.
Certain show commands are supported by the Output Interpreter, which allows you to view an analysis of show command output.
show spantree MST instance active - Use this command to see the MST information on an instance. The addition of the keyword "active" shows only the active ports in the chosen instance.
Show spantree MST mod/port - This command shows you information on the spanning tree state for a specified port when running MST.
show spantree MST configuration - Use this command to see information about the MST configuration on the switch.
Show spantree summary MST - This command provides a quick overview of the MST operation.
Show spantree statistics MST mod/port instance - Use this command to see statistics and other information regarding MST operation on the chosen port.
