Configuring Multiple-STP (MST)

Real World Application & Core Knowledge

If you have read through or completed Labs 4-13 and 4-15 then you should be quite familiar with spanning tree by now and understand that it prevents layer two network loops in a switching topology. This lab will take you’re understanding of spanning tree just a bit further. This lab will explain the function(s) of the IEEE Standard Multiple Spanning Tree protocol also referred to as MST.

MST is commonly used in extremely large networks where running PVST would tax switch resources. Multiple Spanning Tree Protocol gives you the ability to configure a hierarchical spanning tree topology where you have a common spanning tree instance known as the (CIST), technically instance 0 in MST Configuration mode.

MST is designed in a way that you’ll have regional STP instances. A region is defined by the name given in MST configuration mode. For example a region can be Boston or Miami.

Also per each region you have a revision number. This is a locally significant number to signify a revision for the MST configuration. Note that the name, instance map and revision number MUST MATCH in order to build a converged MST topology.

With MST you’re able utilize multiple core switches at a single location (region) by using different instances. For example; core switch 1 is the root bridge for all odd VLANs such as 1, 3, 5, 7 and so on. Core switch 2 is the root bridge for even VLANs such as 2,4,5,8,10 and so on.

Keep in mind when designing a core network for a particular region some VLAN’s may be more traffic intensive, so you may need to further load balance them out across multiple core switch instances in a given region.

MSTP and Rapid-PVST are inter-compatible however only instance 0 (the CIST) is shared from MST to Rapid-PVST.

MSTP configuration is quite different from PVST/Rapid-PVST in such that MSTP has its own configuration mode. In this mode you assign VLANs to a spanning tree instance then you can assign the instance as the root bridge by using the spanning-tree vlan # root primary command.

The port modes remain the same in MSTP as they were in RSTP which are shown below;

In this lab you’ll use SW1 and SW2 to simulate a core and SW3 will be a distribution switch. SW1 will be the root bridge for VLANs 1,3,5,7,9 and SW2 will be the root bridge for VLANs 2,4,6,8,10. SW3 will be running Rapid-PVST.

In this lab you’ll familiarize yourself with the following NEW commands;

Lab Prerequisites

Lab Objectives

Lab Instruction

Step 1. – Configure SW1 to run MST using the revision number 1 and region name REGION1 then configure VLANs 1, 3, 5, 7 and 9 to run on SW1 MST instance 1. Configure SW1 as the root bridge of VLANs 1, 3, 5, 7 and 9 using the static priority of 8192.

To configure SW1 to run MSTP you’ll use the spanning-tree mode mst in global configuration mode. To configure MST instances on SW1 first you need to navigate to MST configuration mode by using the spanning-tree mst configuration command then the instance # vlan # command to create a new instance and map VLANs to that instance. The root bridge is configured on a per instance basis by using the command spanning-tree mst instance# priority # as shown below;

SW1 con0 is now available



Press RETURN to get started.



SW1>enable
SW1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
SW1(config)#spanning-tree mode mst
SW1(config)#spanning-tree mst configuration
SW1(config-mst)#revision 1
SW1(config-mst)#name REGION1
SW1(config-mst)#instance 1 vlan 1,3,5,7,9
SW1(config-mst)#exit
SW1(config)#spanning-tree mst 1 priority 8192
SW1(config)#end
%SYS-5-CONFIG_I: Configured from console by console
SW1#

Step 2. – Configure SW1 to run VLANs 2, 4, 6, 8 and 10 on MST instance 2 then configure SW1 as the secondary root bridge for VLANs 2, 4, 6, 8 and 10 using the static priority of 16384.

To complete this step you’ll use the same commands as previously shown in the step 1 configuration example and as shown below;

SW1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
SW1(config)#spanning-tree mst configuration
SW1(config-mst)#instance 2 vlan 2,4,6,8,10
SW1(config-mst)#exit
SW1(config)#spanning-tree mst 2 priority 16384
SW1(config)#end
%SYS-5-CONFIG_I: Configured from console by console
SW1#

Step 3 – Configure SW2 to run MST using the revision number 1 and region name REGION1 then configure VLANs 1, 3, 5, 7 and 9 to run on SW2 MST instance 1. Configure SW2 the secondary root bridge of VLANs 1, 3, 5, 7 and 9 using the static priority of 16384.

Now mirror the inverse configuration that you’ve done in steps 1 and 2 on SW2 in steps 3 and 4 as shown below;

SW2 con0 is now available



Press RETURN to get started.



SW2>enable
SW2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
SW2(config)#spanning-tree mode mst
SW2(config)#spanning-tree mst configuration
SW2(config-mst)#revision 1
SW2(config-mst)#name REGION1
SW2(config-mst)#instance 1 vlan 1,3,5,7,9
SW2(config-mst)#exit
SW2(config)#spanning-tree mst 1 priority 16384
SW2(config)#end
%SYS-5-CONFIG_I: Configured from console by console
SW2#

Step 4. – Configure SW2 to run VLANs 2, 4, 6, 8 and 10 on MST instance 2 then configure SW2 as the root bridge for VLANs 2, 4, 6, 8 and 10 using the static priority of 8192.

SW2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
SW2(config)#spanning-tree mst configuration
SW2(config-mst)#instance 2 vlan 2,4,6,8,10
SW2(config-mst)#exit
SW2(config)#spanning-tree instance 2 priority 8192
SW2(config)#end
%SYS-5-CONFIG_I: Configured from console by console
SW2#

Step 5. – Configure SW3 to run MST using the revision number 1 and region name REGION1 and assign VLANs 1,3,5,7,9 to instance 1, VLANs 2,4,6,8,10 to instance 2. Verify that the MST Root bridges for instance 1 and instance 2 are correct on SW3.

SW3 con0 is now available



Press RETURN to get started.



SW3>enable
SW3#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
SW3(config)#spanning-tree mode mst
SW3(config)#spanning-tree mst configuration
SW3(config-mst)#revision 1
SW3(config-mst)#name REGION1
SW3(config-mst)#instance 1 vlan 1,3,5,7,9
SW3(config-mst)#instance 2 vlan 2,4,6,8,10
SW3(config-mst)#end
SW3#

To verify that SW1 and SW2 are the root bridges for their respected VLANs by using the show spanning-tree mst # command for instance 1 and 2 as shown below;

SW3#show spanning-tree mst 1,2

##### MST1    vlans mapped:   1,3,5,7,9
Bridge        address 0014.a964.2e00  priority      32769 (32768 sysid 1)
Root          address 0014.f2d2.4180  priority      8193  (8192 sysid 1)
              port    Fa0/10          cost          200000    rem hops 19

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- -----------------------------
Fa0/10           Root FWD 200000    128.10   P2p 
Fa0/15           Altn BLK 200000    128.15   P2p 

##### MST2    vlans mapped:   2,4,6,8,10
Bridge        address 0014.a964.2e00  priority      32770 (32768 sysid 2)
Root          address 001c.57d8.9000  priority      8194  (8192 sysid 2)
              port    Fa0/15          cost          200000    rem hops 19

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- -----------------------------
Fa0/10           Altn BLK 200000    128.10   P2p 
Fa0/15           Root FWD 200000    128.15   P2p 

SW3#

As shown above you can see that MST1 (instance 1) has VLANs 1,3,5,7,9 mapped to it and the root bridge has a priority of 8193 and the MAC address of 0014.f2d2.4180. The root port is specified as Fa0/10 which is directly connected to SW1.

VLANs 2,4,6,8,10 are mapped to MST2 (Instance 2) and the root bridge has a priority of 8193 and the MAC address of 001c.57d8.9000. The root port specified for MST2 is Fa0/15 which is directly connected to SW2.