Multiple Spanning Tree Protocol

The Multiple Spanning Tree Protocol (MSTP) is an STP variant that allows multiple and independent spanning trees to be created over the same physical network. The parameters for each spanning tree can be configured separately, so as to cause a different network devices to be selected as the root bridge or different paths to be selected to form the loop-free topology. Consequently, a given physical interface can be blocked for some of the spanning trees and unblocked for others.

Having set up multiple spanning trees, the set of VLANs in use can be partitioned among them; for example, VLANs 1 - 100 can be assigned to spanning tree 1, VLANs 101 - 200 can be assigned to spanning tree 2, VLANs 201 - 300 can be assigned to spanning tree 3, and so on. Since each spanning tree has a different active topology with different active links, this has the effect of dividing the data traffic among the available redundant links based on the VLAN - a form of load balancing.

Restrictions for configuring MSTP

  • RSTP is not supported. To support RSTP, all vlans are mapped to MSTI 0 when no instance is created for MSTP.

  • PVSTP is not supported.

  • Supports only 16 instances.

  • Untagged EVCs do not participate in MST loop detection.

How to Configure MST Protocol

This section describes the procedure for configuring MSTP:

Enabling Multiple Spanning Tree Protocol

By default, MSTP is disabled on all interfaces. MSTP need not be enabled explicitly on each interfaces. By turning the global configuration on, it is enabled on all interfaces.

Configuring Multiple Spanning Tree Protocol

Describes steps to configure MST

SUMMARY STEPS

  1. configure
  2. spanning-tree mode mst
  3. spanning-tree mst configuration
  4. instance vlan-id vlan vlan-range
  5. name region
  6. revision revision -number
  7. end

DETAILED STEPS

  Command or Action Purpose
Step 1

configure

Example:

Device> configure

Enters global configuration mode.

Step 2

spanning-tree mode mst

Example:

Device> spanning-tree mode mst

Enables MSTP configuration mode.

Step 3

spanning-tree mst configuration

Example:

Device(config)#spanning-tree mst configuration

Enters the MSTP configuration submode.

Step 4

instance vlan-id vlan vlan-range

Example:

Device(config-mstp-inst)# instance 1 vlan 450-480

Maps the VLANs to an MST instance

Step 5

name region

Example:

Device(config-mstp)# name m1

Sets the name of the MSTP region.

Step 6

revision revision -number

Example:

Device(config-mstp)#)revision 1

Sets the revision level of the MSTP region.

Step 7

end

Example:

Device(config-mstp-if)# end

Returns to privileged EXEC mode.

Configuring Untagged EFP over MST Interface

Describes steps to configure untagged EFP over MST:

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. interface interface number
  4. no ip address
  5. service instance number ethernet [name]
  6. bridge-domain bridge-id
  7. encapsulation untagged dot1q {any|vlan-id [,vlan-id [-vlan-d]]}
  8. l2protocol peer stp
  9. end

DETAILED STEPS

  Command or Action Purpose
Step 1

enable

Example:
Router> enable

Enables privileged EXEC mode.

Step 2

configure terminal

Example:
Router# configure terminal

Enters global configuration mode.

Step 3

interface interface number

Example:
Router(config)# interface gigabitEthernet 0/0/5

Specifies the Gigabit Ethernet interface to configure, where: slot/subslot/port-Specifies the location of the interface.

Step 4

no ip address

Example:
Router (config-if)# no ip address

Disables the IP address on the interface.

Step 5

service instance number ethernet [name]

Example:
Router (config-if)#service instance 200 ethernet

Configure an EFP (service instance) and enter service instance configuration mode.

Step 6

bridge-domain bridge-id

Example:
Router (config-if-srv)#  bridge-domain from-encapsulation

Creates a list of bridge domains for an EFP trunk port using the bridge-domain IDs derived from the encapsulation VLAN numbers.

Step 7

encapsulation untagged dot1q {any|vlan-id [,vlan-id [-vlan-d]]}

Example:
Router (config-if-srv)# encapsulation dot1q 20

Configures the encapsulation. Defines the matching criteria that maps the ingress dot1q or untagged frames on an interface for the appropriate service instance.

Step 8

l2protocol peer stp

Example:
Router (config-if-srv)# l2protocol peer stp

Configures STP to peer with a neighbor on a port that has an EFP service instance.

Step 9

end

Example:
Device(config-mstp-if)# end

Returns to privileged EXEC mode.

Configuration Example

This example shows how to configure STP to peer with a neighbor on a service instance. 


interface GigabitEthernet0/0/0
no ip address
negotiation auto
service instance trunk 10 ethernet
  encapsulation dot1q 10-20
  bridge-domain from-encapsulation
!
service instance 1024 ethernet
  encapsulation untagged
  l2protocol peer stp
  bridge-domain 1024
!
end