Multiple Spanning Tree Protocol (MSTP) on Cisco Switch

Multiple Spanning Tree Protocol (MSTP) was first specified in IEEE 802.1s and is standardized in IEEE 802.1Q. MSTP enables multiple VLANs to be mapped to the same spanning-tree instance, reducing the number of spanning-tree instances needed to support a large number of VLANs. MSTP provides multiple forwarding paths for data traffic and enables load balancing. It improves the fault tolerance of the network because a failure in one instance, or forwarding path, does not affect other instances.

MSTP has a backward compatibility mode in which it can fall back to STP or RSTP operation on links with bridges that support only STP or RSTP.

Steps for Configure MST on Cisco Switch.

• Configure all switches to use MST.
• Configure VLAN 10,20,30,40,50 and 60 on all switches.
• Configure VLAN 10 and 20 to use instance 1.
• Configure VLAN 30 and 40 to use instance 2.
• Configure VLAN 50 and 60 to use instance 3.
• Configure SW1 to be the root bridge for instance 1.
• Configure SW2 to be the root bridge for instance 2.
• Configure SW2 to be the root bridge for instance 2.

Topology for MST configuration

Configure Sw1 to run MST use revision 1 and name MYMST the map VLANs 10,20 on MST instance 1, VLAN 30,40 on instance 2 and VLAN 50, 60 on instance 3. Configure SW1 as a root bridge for instance 1.

SW1(config)#spanning-tree mode mst
SW1(config)#spanning-tree mst configuration
SW1(config-mst)#revision 1
SW1(config-mst)#name MYMST
SW1(config-mst)#instance 1 vlan 10,20
SW1(config-mst)#instance 2 vlan 30,40
SW1(config-mst)#instance 3 vlan 50,60
SW1(config-mst)#exit
SW1(config)#spanning-tree mst 1 priority 8192
SW1(config)#spanning-tree mst 2 priority 16384
SW1(config)#spanning-tree mst 3 priority 16384
SW1(config)#end

Configure Sw2 to run MST use revision 1 and name MYMST the map VLANs 10,20 on MST instance 1, VLAN 30,40 on instance 2 and VLAN 50, 60 on instance 3. Configure SW2 as a root bridge for instance 2.

SW2(config)#spanning-tree mode mst
SW2(config)#spanning-tree mst configuration
SW2(config-mst)#revision 1
SW2(config-mst)#name MYMST
SW2(config-mst)#instance 1 vlan 10,20
SW2(config-mst)#instance 2 vlan 30,40
SW2(config-mst)#instance 3 vlan 50,60
SW2(config-mst)#exit
SW2(config)#spanning-tree mst 1 priority 16384
SW2(config)#spanning-tree mst 2 priority 8192
SW2(config)#spanning-tree mst 3 priority 24576
SW2(config)#end

Configure Sw3 to run MST use revision 1 and name MYMST the map VLANs 10,20 on MST instance 1, VLAN 30,40 on instance 2 and VLAN 50, 60 on instance 3. Configure SW3 as a root bridge for instance 3.

SW3(config)#spanning-tree mode mst
SW3(config)#spanning-tree mst configuration
SW3(config-mst)#revision 1
SW3(config-mst)#name MYMST
SW3(config-mst)#instance 1 vlan 10,20
SW3(config-mst)#instance 2 vlan 30,40
SW3(config-mst)#instance 3 vlan 50,60
SW3(config-mst)#exit
SW3(config)#spanning-tree mst 1 priority 24576
SW3(config)#spanning-tree mst 2 priority 24576
SW3(config)#spanning-tree mst 3 priority 8192
SW3(config)#end

To verify that SW1 , SW2 and SW3 are the root bridges for their respected VLANs by using the show spanning-tree mst