Configuration Guide User guide
1172 FastIron Configuration Guide
53-1002494-02
802.1s Multiple Spanning Tree Protocol
• The CIST 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 an L2 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.
Brocade(config)#mstp scope all
Syntax: [no] mstp scope all
NOTE
MSTP is not operational however until the mstp start command is issued as described in “Activating
MSTP on a switch” on page 1178.
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 Brocade
MSTP implementation however, a VLAN ID can be pre-mapped to another MSTI as described in
“Configuring an MSTP instance” on page 1175. A VLAN whose 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.
Using the [no] option 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.
Reduced occurrences of MSTP reconvergence
When a VLAN is deleted, the Brocade device retains the associated VLAN to MSTI mapping instead
of deleting it from the configuration. This way, a VLAN can be pre-mapped to an MSTI and MSTP
reconvergence may not be necessary when a VLAN is added to or deleted from the configuration.
As long as the VLAN being created or deleted is pre-mapped to an MSTI, and the VLAN to MSTI
mapping has not changed, MSTP reconvergence will not occur.
NOTE
MSTP reconvergence occurs when the VLAN to MSTI mapping is changed using the mstp instance
command.
You can optionally remove VLAN to MSTI mappings from the configuration. Refer to “Deleting a
VLAN to MSTI mapping” on page 1173.
The following shows an example application.
Example application of MSTP reconvergence
The following example shows the running configuration file before and after deleting a VLAN from
the configuration. The VLAN to MSTI mapping is retained in the running configuration, even after
the VLAN is deleted.