Configuring MSTP Mode and Scope

With the introduction of MSTP, a system can be either under MSTP mode or not under MSTP mode. The default state is to not be under MSTP mode. MSTP configuration can only be performed in a system under MSTP mode.

With a system configured under MSTP mode, there is a concept called MSTP scope. MSTP scope defines the VLANs that are under direct MSTP control. You cannot run 802.1D or 802.1w on any VLAN (even outside of MSTP scope) and you cannot create topology groups when a system is under MSTP mode. While a VLAN group will still be supported when a system is under MSTP mode, the member VLAN should either be all in the MSTP scope or all out of the MSTP scope.

When a system is configured from non-MSTP mode to MSTP mode, the following changes are made to the system configuration:

  • All 802.1D and 802.1w STP instances are deleted regardless of whether the VLAN is inside the MSTP scope or not.
  • All topology groups are deleted.
  • Any VSRP configuration is deleted.
  • Single-span (if configured) is deleted.
  • MRP running on a VLAN inside MSTP scope is deleted.
  • The common and internal spanning trees (CIST) collection is created and all VLANS inside the MSTP scope are attached with the CIST.

Make sure that no physical Layer 2 loops exist prior to switching from non-MSTP mode to MSTP mode. If, for example, you have a Layer 2 loop topology configured as a redundancy mechanism before you perform the switch, a Layer 2 storm should be expected.

To configure a system into MSTP mode, use the following command at the Global Configuration level. 

device(config)# mstp scope all
Note: When the mstp scope all command is issued, the ports associated to VLANs in which any STP is configured transition to the blocking state. You must enter the mstp start command to re-converge the ports, VLANs, and VEs.

Once the system is configured into MSTP mode, CIST (sometimes referred to as “instance 0”) is created and all existing VLANs inside the MSTP scope are controlled by CIST. In addition, whenever you create a new VLAN inside MSTP scope, it is put under CIST control by default. In the RUCKUS ICX MSTP implementation however, a VLAN ID can be pre-mapped to another MSTI as described in the “Configuring an MSTP instance” section. A VLAN for which the ID is pre-mapped, will attach to the specified MSTI instead of to the CIST when created.

Note: Once under MSTP mode, CIST always controls all ports in the system. If you do not want a port to run MSTP, configure the no spanning-tree command under the specified interface configuration.

Configuring no spanning-tree command on a system that is configured for MSTP mode changes the system to non-MSTP mode. When this switch is made, all MSTP instances are deleted together with all MSTP configurations. ALL VLANs inside the original MSTP scope will not run any Layer 2 protocols after the switch.

Parent topic: 802.1s Multiple Spanning Tree Protocol