Configuring Multiple Spanning Tree Protocol (MSTP)

Multiple Spanning Tree Protocol as defined in IEEE802.1s or IEEE802.1Q-2005 provides the ability to manage multiple VLANs using Multiple Spanning Tree Instances (MSTIs). MSTP allows the formation of MST regions that can run multiple MSTIs. MSTP regions and other STP bridges are interconnected using the Common and Internal Spanning Tree (CIST).

MSTP regions are defined to be a collection of switches that have the same VLANs configured on all of them. All switches in a region must have the same configuration name, revision level, VLANs, and VLAN to MSTI associations. Each MST region may have multiple MSTIs operating within it but an MSTI cannot span multiple regions. Each MSTI must have a regional root which it may or may not share with another MSTI.

The CIST is the default spanning tree instance in MSTP. CIST forms a larger spanning tree for the entire bridged network by combining MSTP regions and single-instance spanning trees. The CIST instance has an MSTI ID of 0 which cannot be deleted or changed. When a new port-based or tagged VLAN is created, it is associated with the CIST by default and is automatically given an MSTI ID of 0. The default VLAN on a switch is also associated with the CIST. When a VLAN is assigned to an MSTI, it partially remains a member of the CIST. This is because CIST is used by MSTP to enable communication with MSTP regions and RSTP/STP single-instances in the network. CIST also has regional roots.

The switch with the lowest CIST priority value functions as the root bridge for all the MSTP regions and STP/ RSTP single-instance spanning trees in the network.

The following commands support the configuration of MST instances on a local switch:

CLI (network-admin@Leaf1) > mst-config-create

instance-id 0..64	Specify the ID as a number between 1 and 64 for MST configuration. MST ID 0 corresponds to the CIST instance.
vlans vlan-list	Specify the list of VLANs associated with the MST configuration
bridge-priority 0..61440	Specify the bridge priority as a number between 0 and 61440 which is a multiple of 4096. For example, the values can be 0, 4096, 8192, up to 61440. The default value is 32768.

Once created, the MST instance can be modified by the command:

CLI (network-admin@Leaf1) > mst-config-modify

instance-id 0..64	Specify the ID as a number between 1 and 64 for MST configuration. MST ID 0 corresponds to the CIST instance.
Specify one or more of the following options:
vlans vlan-list	Specify the list of VLANs associated with the MST configuration.
bridge-priority 0..61440	Specify the bridge priority as a number between 0 and 61440 which is a multiple of 4096. For example, the values can be 0, 4096, 8192, up to 61440. The default value is 32768.

The MST configuration can be deleted using the command:

CLI (network-admin@Leaf1) > mst-config-delete

instance-id 0..64

Specify the MST instance ID that needs to be deleted.

The current MST configuration on the switch can be viewed using the command:

CLI (network-admin@Leaf1) > mst-config-show

instance-id 0..64	Specify the ID as a number between 1 and 64 for MST configuration. MST ID 0 corresponds to the CIST instance.
Specify one or more of the following options to view the information related to those options.
vlans vlan-list	Specify the list of VLANs associated with the MST configuration.
bridge-priority 0..61440	Specify the bridge priority as a number between 0 and 61440.