Configuring MSTP (Cont.)

These sections contain this configuration information

•Configuring the Switch Priority (optional)

•Configuring the Hello Time (optional)

•Configuring the Forwarding-Delay Time (optional)

•Configuring the Maximum-Aging Time (optional)

•Configuring the Maximum-Hop Count (optional)

•Specifying the Link Type to Ensure Rapid Transitions (optional)

•Designating the Neighbor Type (optional)

•Restarting the Protocol Migration Process (optional)

Configuring the Switch Priority

You can configure the switch priority and make it more likely that a standalone switch or a switch in the stack will be chosen as the root switch.

Stacking is supported only on Catalyst 2960-S switches running the LAN base image.

          Exercise care when using this command. For most situations, we recommend that you use the spanning-tree mst instance-id root primary and thespanning-tree mst instance-id root secondary global configuration commands to modify the switch priority.

         Beginning in privileged EXEC mode, follow these steps to configure the switch priority. This procedure is optional.

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	spanning-tree mstinstance-id prioritypriority	Configure the switch priority. •For instance-id, you can specify a single instance, a range of instances separated by a hyphen, or a series of instances separated by a comma. The range is 0 to 4094. •For priority, the range is 0 to 61440 in increments of 4096; the default is 32768. The lower the number, the more likely the switch will be chosen as the root switch. Priority values are 0, 4096, 8192, 12288, 16384, 20480, 24576, 28672, 32768, 36864, 40960, 45056, 49152, 53248, 57344, and 61440. All other values are rejected.
Step 3	end	Return to privileged EXEC mode.
Step 4	show spanning-tree mst instance-id	Verify your entries.
Step 5	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To return the switch to its default setting, use the no spanning-tree mst instance-id priority global configuration command.

Configuring the Hello Time

You can configure the interval between the generation of configuration messages by the root switch by changing the hello time.

Beginning in privileged EXEC mode, follow these steps to configure the hello time for all MST instances. This procedure is optional.

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	spanning-tree mst hello-timeseconds	Configure the hello time for all MST instances. The hello time is the interval between the generation of configuration messages by the root switch. These messages mean that the switch is alive. For seconds, the range is 1 to 10; the default is 2.
Step 3	end	Return to privileged EXEC mode.
Step 4	show spanning-tree mst	Verify your entries.
Step 5	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To return the switch to its default setting, use the no spanning-tree mst hello-time global configuration command.

Configuring the Forwarding-Delay Time

Beginning in privileged EXEC mode, follow these steps to configure the forwarding-delay time for all MST instances. This procedure is optional.

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	spanning-tree mst forward-time seconds	Configure the forward time for all MST instances. The forward delay is the number of seconds a port waits before changing from its spanning-tree learning and listening states to the forwarding state. For seconds, the range is 4 to 30; the default is 15.
Step 3	end	Return to privileged EXEC mode.
Step 4	show spanning-tree mst	Verify your entries.
Step 5	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To return the switch to its default setting, use the no spanning-tree mst forward-time global configuration command.

Configuring the Maximum-Aging Time

Beginning in privileged EXEC mode, follow these steps to configure the maximum-aging time for all MST instances. This procedure is optional.

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	spanning-tree mst max-ageseconds	Configure the maximum-aging time for all MST instances. The maximum-aging time is the number of seconds a switch waits without receiving spanning-tree configuration messages before attempting a reconfiguration. For seconds, the range is 6 to 40; the default is 20.
Step 3	end	Return to privileged EXEC mode.
Step 4	show spanning-tree mst	Verify your entries.
Step 5	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To return the switch to its default setting, use the no spanning-tree mst max-age global configuration command.

Configuring the Maximum-Hop Count

Beginning in privileged EXEC mode, follow these steps to configure the maximum-hop count for all MST instances. This procedure is optional.

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	spanning-tree mst max-hops hop-count	Specify the number of hops in a region before the BPDU is discarded, and the information held for a port is aged. For hop-count, the range is 1 to 255; the default is 20.
Step 3	end	Return to privileged EXEC mode.
Step 4	show spanning-tree mst	Verify your entries.
Step 5	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To return the switch to its default setting, use the no spanning-tree mst max-hops global configuration command.

Specifying the Link Type to Ensure Rapid Transitions

If you connect a port to another port through a point-to-point link and the local port becomes a designated port, the RSTP negotiates a rapid transition with the other port by using the proposal-agreement handshake to ensure a loop-free topology 

By default, the link type is controlled from the duplex mode of the interface: a full-duplex port is considered to have a point-to-point connection; a half-duplex port is considered to have a shared connection. If you have a half-duplex link physically connected point-to-point to a single port on a remote switch running MSTP, you can override the default setting of the link type and enable rapid transitions to the forwarding state.

Beginning in privileged EXEC mode, follow these steps to override the default link-type setting. This procedure is optional.

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	interfaceinterface-id	Specify an interface to configure, and enter interface configuration mode. Valid interfaces include physical ports, VLANs, and port-channel logical interfaces. The VLAN ID range is 1 to 4094. The port-channel range is 1 to 6.
Step 3	spanning-tree link-type point-to-point	Specify that the link type of a port is point-to-point.
Step 4	end	Return to privileged EXEC mode.
Step 5	show spanning-tree mst interfaceinterface-id	Verify your entries.
Step 6	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To return the port to its default setting, use the no spanning-tree link-type interface configuration command.

Designating the Neighbor Type

A topology could contain both prestandard and IEEE 802.1s standard compliant devices. By default, ports can automatically detect prestandard devices, but they can still receive both standard and prestandard BPDUs. When there is a mismatch between a device and its neighbor, only the CIST runs on the interface.

You can choose to set a port to send only prestandard BPDUs. The prestandard flag appears in all the show commands, even if the port is in STP compatibility mode.

Beginning in privileged EXEC mode, follow these steps to override the default link-type setting. This procedure is optional.

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	interface interface-id	Specify an interface to configure, and enter interface configuration mode. Valid interfaces include physical ports.
Step 3	spanning-tree mst pre-standard	Specify that the port can send only prestandard BPDUs.
Step 4	end	Return to privileged EXEC mode.
Step 5	show spanning-tree mst interface interface-id	Verify your entries.
Step 6	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To return the port to its default setting, use the no spanning-tree mst prestandard interface configuration command.

Restarting the Protocol Migration Process

A switch running MSTP supports a built-in protocol migration mechanism that enables it to interoperate with legacy IEEE 802.1D switches. If this switch receives a legacy IEEE 802.1D configuration BPDU (a BPDU with the protocol version set to 0), it sends only IEEE 802.1D BPDUs on that port. An MSTP switch also can detect that a port is at the boundary of a region when it receives a legacy BPDU, an MST BPDU (Version 3) associated with a different region, or an RST BPDU (Version 2).

However, the switch does not automatically revert to the MSTP mode if it no longer receives IEEE 802.1D BPDUs because it cannot detect whether the legacy switch has been removed from the link unless the legacy switch is the designated switch. A switch also might continue to assign a boundary role to a port when the switch to which it is connected has joined the region.

To restart the protocol migration process (force the renegotiation with neighboring switches) on the switch, use the clear spanning-tree detected-protocols privileged EXEC command.

To restart the protocol migration process on a specific interface, use the clear spanning-tree detected-protocols interface interface-id privileged EXEC command.

Displaying the MST Configuration and Status

To display the spanning-tree status, use one or more of the privileged EXEC commands in Table 18-5:

Table 18-5 Commands for Displaying MST Status 

Command	Purpose
show spanning-tree mst configuration	Displays the MST region configuration.
show spanning-tree mst configuration digest	Displays the MD5 digest included in the current MSTCI.
show spanning-tree mst instance-id	Displays MST information for the specified instance.
show spanning-tree mst interface interface-id	Displays MST information for the specified interface.