4.1.2 Lab – Implement MST Answers

4.1.2 Lab – Implement MST Answers

Lab – Implement MST (Answers Version)

Answers Note: Red font color or gray highlights indicate text that appears in the instructor copy only.

Topology

This topology has 3 switches and a PC. A1 F0/1 is connected to D1 G1/0/5 and A1 F0/2 is connected to D1 G1/0/6. A1 F0/3 is connected to D1 G1/0/5 and A1 F0/4 is connected to D1 G1/0/6. D1 g1/0/1 is connected D2 G1/0/1. PC 3 is connected A1 F0/23.

Objectives

Part 1: Build the Network and Configure Basic Device Settings and Interface Addressing

Part 2: Implement and Observe MST

Part 3: Configure, Tune and Verify Basic MST Operation

Background / Scenario

Cisco’s Per VLAN Spanning Tree (PVST+) provides a significant step up from standard spanning tree in terms of flexibility. PVST+ allows each VLAN to have its own independent spanning tree, thereby making better use of available links in the network. A drawback to PVST+ is that there is an instance of PVST+ running for each VLAN in the network, regardless of whether there are different spanning-tree topologies required. This presents the potential for overwhelming the switch CPU and memory. Additionally, Cisco switches support a finite number of PVST instances. If more VLANs are created than there are PVST+ instances supported on a particular switch, some of the VLANs will not have any STP running, and therefore not have any switching loop protection. PVST+ and Rapid PVST+ are simply unusable in that kind of environment. Lastly, PVST+ and Rapid PVST+ are Cisco-proprietary protocols and generally unusable in mixed vendor environments.

Cisco was involved in the early development of Multiple Spanning Tree. MST was standardized as IEEE 802.1s in 2002 and merged into 802.1Q in 2005. MST is an open protocol derived from RSTP, sharing all its rapid convergence properties, and in fact, the only standardized spanning-tree protocol for VLAN-based networks supported by multiple vendors. MST is a compromise between common spanning-tree and per-VLAN spanning tree. An MST instance represents a unique spanning-tree topology. Multiple MST instances can be created to account for each of the required spanning-tree topologies in a network, and an arbitrary number of VLANs can be mapped to a single MST instance.

In this lab, you will set up two instances of MST, one for VLANs 2 and 3 and the other for VLANs 4 and 5. All other VLANs will be mapped to the default MST instance (also referred to as IST or Internal Spanning Tree).

Note: This lab is an exercise in deploying and verifying MST and does not reflect networking best practices.

Note: The switches used with CCNP hands-on labs are Cisco 3650 with Cisco IOS XE release 16.9.4 (universalk9 image) and Cisco 2960+ with IOS release 15.2 (lanbase image). Other routers and Cisco IOS versions can be used. Depending on the model and Cisco IOS version, the commands available and the output produced might vary from what is shown in the labs.

Note: Ensure that the switches have been erased and have no startup configurations. If you are unsure contact your instructor.

Answers Note: Refer to the Answers Lab Manual for the procedures to initialize and reload devices.

Required Resources

  • 2 Switches (Cisco 3650 with Cisco IOS XE release 16.9.4 universal image or comparable)
  • 1 Switch (Cisco 2960+ with Cisco IOS release 15.2 lanbase image or comparable)
  • 1 PC (Windows with a terminal emulation program, such as Tera Term)
  • Console cables to configure the Cisco IOS devices via the console ports
  • Ethernet cables as shown in the topology

Instructions

Part 1:Build the Network and Configure Basic Device Settings and Interface Addressing

In Part 1, you will set up the network topology and configure basic settings.

Step 1:Cable the network as shown in the topology.

Attach the devices as shown in the topology diagram, and cable as necessary.

Step 2:Configure basic settings for each switch.

  1. Console into each switch, enter global configuration mode, and apply the basic settings. The startup configurations for each device are provided below.

Switch D1

hostname D1

banner motd # D1, Multiple Spanning Tree #

spanning-tree mode rapid-pvst

line con 0

exec-timeout 0 0

logging synchronous

exit

interface range g1/0/1-24, g1/1/1-4, g0/0

shutdown

exit

interface range g1/0/1, g1/0/5-6

switchport mode trunk

no shutdown

exit

vlan 2

name SecondVLAN

exit

vlan 3

name ThirdVLAN

exit

vlan 4

name FourthVLAN

exit

vlan 5

name FifthVLAN

exit

Switch D2

hostname D2

banner motd # D2, Multiple Spanning Tree #

spanning-tree mode rapid-pvst

line con 0

exec-timeout 0 0

logging synchronous

exit

interface range g1/0/1-24, g1/1/1-4, g0/0

shutdown

exit

interface range g1/0/1, g1/0/5-6

switchport mode trunk

no shutdown

exit

vlan 2

name SecondVLAN

exit

vlan 3

name ThirdVLAN

exit

vlan 4

name FourthVLAN

exit

vlan 5

name FifthVLAN

exit

Switch A1

hostname A1

banner motd # A1, Multiple Spanning Tree #

spanning-tree mode rapid-pvst

line con 0

exec-timeout 0 0

logging synchronous

exit

interface range f0/1-24, g0/1-2

shutdown

exit

interface range f0/1-4

switchport mode trunk

no shutdown

interface f0/23

no shutdown

exit

vlan 2

name SecondVLAN

exit

vlan 3

name ThirdVLAN

exit

vlan 4

name FourthVLAN

exit

vlan 5

name FifthVLAN

exit

  1. Set the clock on each switch to UTC time.
  2. Save the running configuration to startup-config.

Note: Outputs and Spanning Tree topologies highlighted in this lab may be different than what you observe using your own equipment. It is critically important for you to understand how Spanning Tree makes its decisions, and how those decisions impact the operational topology of the network.

Part 2:Implement and Observe MST

Step 1:Configure MST on D1 and D2.

In this step we will configure MST on D1 and D2 only. We will do this so we can observe their interaction with each other as well as their interaction with A1.

Open configuration window

  1. On D1 and D2, issue the command spanning-tree mode mst.

D1(config)# spanning-tree mode mst

  1. At this point, with no MST-specific configuration, MST Instance 0 is operational for all VLANs. Issue the command show spanning-tree and you will see in the output that the spanning tree information is about MST 0. Issue the command show spanning-tree mst and you will see the MST-specific STP information that is specific to MST 0 only. Take note of the information displayed for interfaces g1/0/5 and g1/0/6 because they are connected to a switch that is not running MST. Their type is listed as P2p Bound (PVST).

D1# show spanning-tree

MST0

Spanning tree enabled protocol mstp

Root IDPriority32768

Addressd8b1.9028.af80

This bridge is the root

Hello Time2 secMax Age 20 secForward Delay 15 sec

Bridge IDPriority32768(priority 32768 sys-id-ext 0)

Addressd8b1.9028.af80

Hello Time2 secMax Age 20 secForward Delay 15 sec

InterfaceRole Sts CostPrio.Nbr Type

——————- —- — ——— ——– ——————————–

Gi1/0/1Desg FWD 20000128.1P2p

Gi1/0/5Desg FWD 200000128.5P2p Bound(PVST)

Gi1/0/6Desg FWD 200000128.6P2p Bound(PVST)

D1# show spanning-tree mst

##### MST0vlans mapped:1-4094

Bridgeaddress d8b1.9028.af80priority32768 (32768 sysid 0)

Rootthis switch for the CIST

Operationalhello time 2 , forward delay 15, max age 20, txholdcount 6

Configuredhello time 2 , forward delay 15, max age 20, max hops20

InterfaceRole Sts CostPrio.Nbr Type

—————-—- — ——— ——– ———————-

Gi1/0/1Desg FWD 20000128.1P2p

Gi1/0/5Desg FWD 200000128.5P2p Bound(PVST)

Gi1/0/6Desg FWD 200000128.6P2p Bound(PVST)

Close configuration window

  1. The basic behavior of MST is the same as spanning tree, where a root bridge must be selected, then root ports, and finally best paths to the root bridge from all non-root bridges. In the current network, we can see that D1 has been elected the root bridge. The bridge priority defaults to 32768, so the election is based on D1 having a lower base MAC address. The switch elected as root may be different in your lab topology, but the rules for election remain the same. Issue the command show spanning-tree root on switch A1. Switch A1 is running five instances of spanning tree.

Open configuration window

A1# show spanning-tree root

RootHello Max Fwd

VlanRoot IDCostTimeAge DlyRoot Port

—————- ——————– ——— —– — ————–

VLAN000132768 d8b1.9028.af801922015Fa0/1

VLAN000232768 d8b1.9028.af801922015Fa0/1

VLAN000332768 d8b1.9028.af801922015Fa0/1

VLAN000432768 d8b1.9028.af801922015Fa0/1

VLAN000532768 d8b1.9028.af801922015Fa0/1

  1. Issue the show spanning-tree root command on D2 and the output will be different. This is because with MST, only one instance of the spanning-tree algorithm runs, regardless of the number of VLANs mapped to it.

D2# show spanning-tree root

RootHello Max Fwd

MST InstanceRoot IDCostTimeAge DlyRoot Port

—————- ——————– ——— —– — ————–

MST032768 d8b1.9028.af80022015Gi1/0/1

  1. Configure A1 to use MST.

A1(config)# spanning-tree mode mst

Close configuration window

Part 3:Configure, Tune and Verify Basic MST Operation

In the last part, you configured all three switches to run MST. In this part, you will further configure, tune, and verify MST to support the unique topological requirements.

Step 1:Create and verify an MST configuration.

MST allows network engineers to reduce the load of the spanning-tree protocol while still providing unique spanning-tree topologies for groups of VLANs. MST configuration must be completed by hand on each switch individually.

Complete the following tasks on switch D1:

Open configuration window

  1. Enter MST configuration mode using the command spanning-tree mst configuration.

D1(config)# spanning-tree mst configuration

  1. Configure an MST region name. Our example will be CCNPv8.

D1(config-mst)# name CCNPv8

  1. Configure an MST configuration revision number. Our example will be 1.

D1(config-mst)# revision 1

  1. Configure instance 1 to include VLAN 2.

D1(config-mst)# instance 1 vlan 2

  1. Configure instance 2 to include VLAN 4.

D1(config-mst)# instance 2 vlan 4

  1. Commit the configuration by typing exit and returning to global configuration mode.

D1(config-mst)# exit

D1(config)# end

  1. Issue the command show spanning-tree mst to verify the configuration is in place.

Note: While in spanning-tree mst configuration mode, you can use the show current and show pending commands to view current and pending configuration settings.

D1# show spanning-tree mst

##### MST0vlans mapped:1,3,5-4094

Bridgeaddress d8b1.9028.af80priority32768 (32768 sysid 0)

Rootthis switch for the CIST

Operationalhello time 2 , forward delay 15, max age 20, txholdcount 6

Configuredhello time 2 , forward delay 15, max age 20, max hops20

InterfaceRole Sts CostPrio.Nbr Type

—————-—- — ——— ——– ————-

Gi1/0/1Desg FWD 20000128.1P2p

Gi1/0/5Desg FWD 200000128.5P2p

Gi1/0/6Desg FWD 200000128.6P2p

##### MST1vlans mapped:2

Bridgeaddress d8b1.9028.af80priority32769 (32768 sysid 1)

Rootthis switch for MST1

InterfaceRole Sts CostPrio.Nbr Type

—————-—- — ——— ——– ————-

Gi1/0/1Desg FWD 20000128.1P2p

Gi1/0/5Desg FWD 200000128.5P2p

Gi1/0/6Desg FWD 200000128.6P2p

##### MST2vlans mapped:4

Bridgeaddress d8b1.9028.af80priority32770 (32768 sysid 2)

Rootthis switch for MST2

InterfaceRole Sts CostPrio.Nbr Type

—————-—- — ——— ——– ————-

Gi1/0/1Desg FWD 20000128.1P2p

Gi1/0/5Desg FWD 200000128.5P2p

Gi1/0/6Desg FWD 200000128.6P2p

Close configuration window

  1. This configuration does not propagate to other switches. Each switch exchanges digest information summarizing the VLAN-to-Instance mappings it has configured. If a switch receives a BPDU with a different digest, it assumes that the sender is in a different MST region. The output below is what A1 shows in the topology used to create this lab. Notice that the ports connected to D1 are classified as P2p Bound (RSTP).

Open configuration window

A1# show spanning-tree mst

##### MST0vlans mapped:1-4094

Bridgeaddress f078.1647.4580priority32768 (32768 sysid 0)

Rootaddress d8b1.9028.af80priority32768 (32768 sysid 0)

portFa0/3path cost20000

Regional Root address d8b1.905d.c300priority32768 (32768 sysid 0)

internal cost 200000rem hops 19

Operationalhello time 2 , forward delay 15, max age 20, txholdcount 6

Configuredhello time 2 , forward delay 15, max age 20, max hops20

InterfaceRole Sts CostPrio.Nbr Type

—————- —- — ——— ——– —————————–

Fa0/1Altn BLK 200000128.1P2p Bound(RSTP)

Fa0/2Altn BLK 200000128.2P2p Bound(RSTP)

Fa0/3Root FWD 200000128.3P2p

Fa0/4Altn BLK 200000128.4P2p

Fa0/23Desg FWD 2000000128.23P2p

A1# show spanning-tree mst configuration digest

Name[]

Revision0Instances configured 1

Digest0xAC36177F50283CD4B83821D8AB26DE62

Pre-std Digest0xBB3B6C15EF8D089BB55ED10D24DF44DE

Close configuration window

And here is the digest from D1:

Open configuration window

D1# show spanning-tree mst configuration digest

Name[CCNPv8]

Revision1Instances configured 3

Digest0xB41829F9030A054FB74EF7A8587FF58D

Pre-std Digest0x6C5B0D56DA63408ED96AA5F1C5356F98

As you can see here, the digest values are different.

  1. Configure MST on D1, change the revision number to 2 and add VLAN 3 to instance 1 and VLAN 5 to instance 2.

D1(config)# spanning-tree mst configuration

D1(config-mst)#show current

Current MST configuration

Name[CCNPv8]

Revision1Instances configured 3

InstanceVlans mapped

——–———————————————————————

01,3,5-4094

12

24

——————————————————————————-

D1(config-mst)# revision 2

D1(config-mst)# instance 1 vlan 3

D1(config-mst)# instance 2 vlan 5

D1(config-mst)# show pending

Pending MST configuration

Name[CCNPv8]

Revision2Instances configured 3

InstanceVlans mapped

——–———————————————————————

01,6-4094

12-3

24-5

——————————————————————————-

D1(config-mst)# exit

D1(config)# end

Close configuration window

  1. Now configure D2 and A1 with the same configuration settings (name CCNPv8, revision2, instance 1 vlans 2-3, instance 2 vlans 4-5). After completing the configuration on D2 and A1, the output of show spanning-tree mst on A1 should be similar to the following output. D1 in this case is the root for all 3 instances.

Open configuration window

A1# show spanning-tree mst

##### MST0vlans mapped:1,6-4094

Bridgeaddress f078.1647.4580priority32768 (32768 sysid 0)

Rootaddress d8b1.9028.af80priority32768 (32768 sysid 0)

portFa0/1path cost0

Regional Root address d8b1.9028.af80priority32768 (32768 sysid 0)

internal cost 200000rem hops 19

Operationalhello time 2 , forward delay 15, max age 20, txholdcount 6

Configuredhello time 2 , forward delay 15, max age 20, max hops20

InterfaceRole Sts CostPrio.Nbr Type

—————- —- — ——— ——– —————————–

Fa0/1Root FWD 200000128.1P2p

Fa0/2Altn BLK 200000128.2P2p

Fa0/3Altn BLK 200000128.3P2p

Fa0/4Altn BLK 200000128.4P2p

Fa0/23Desg FWD 2000000128.23P2p

##### MST1vlans mapped:2-3

Bridgeaddress f078.1647.4580priority32769 (32768 sysid 1)

Rootaddress d8b1.9028.af80priority32769 (32768 sysid 1)

portFa0/1cost200000rem hops 19

InterfaceRole Sts CostPrio.Nbr Type

—————- —- — ——— ——– —————————–

Fa0/1Root FWD 200000128.1P2p

Fa0/2Altn BLK 200000128.2P2p

Fa0/3Altn BLK 200000128.3P2p

Fa0/4Altn BLK 200000128.4P2p

Fa0/23Desg FWD 2000000128.23P2p

##### MST2vlans mapped:4-5

Bridgeaddress f078.1647.4580priority32770 (32768 sysid 2)

Rootaddress d8b1.9028.af80priority32770 (32768 sysid 2)

portFa0/1cost200000rem hops 19

InterfaceRole Sts CostPrio.Nbr Type

—————- —- — ——— ——– —————————–

Fa0/1Root FWD 200000128.1P2p

Fa0/2Altn BLK 200000128.2P2p

Fa0/3Altn BLK 200000128.3P2p

Fa0/4Altn BLK 200000128.4P2p

Fa0/23Desg FWD 2000000128.23P2p

Step 2:Controlling the Root Bridge.

Just like with PVST+ and Rapid PVST+, the current root bridge was elected based on the lowest Bridge ID (consisting of the Priority, extended system ID equal to the VLAN ID, and base MAC address values).

With the priority and extended system IDs being identical, the root bridge’s MAC is numerically smaller than the local bridge’s MAC. The result is that in a completely un-configured network, one single switch will be elected as the root bridge. The resulting choice of switch may or may not be desirable.

With PVST+ or Rapid PVST+, root bridge selection is done for each VLAN. With MST, the root bridge is based on instances.

There are two basic ways to manipulate the configuration to control the location of the root bridge:

  • The spanning-tree mst instance-id priority value command can be used to manually set a priority value
  • The spanning-tree mst instance-id root { primary | secondary } command can be used to automatically set a priority value.

The difference between the two is the priority command will set a specific number (multiple of 4096) as the priority, while the root primary command will set the local bridge’s priority to 24,576 (if the local bridge MAC is lower than the current root bridge’s MAC) or 4096 lower than the current root’s priority (if the local bridge MAC is higher than the current root bridge’s MAC).

The logic behind this operation is straight-forward. The root primary command tries to lower the priority only as much as is needed to win the root election, while leaving priorities between 24576 and the default 32768 for use by secondary bridges. The command always takes the entire Bridge ID into account when computing the resulting priority value.

The spanning-tree mst instance-id secondary command will statically set the local bridge’s priority to 28,672. In an otherwise unconfigured network where all switch priorities default to 32,768, the root primary command will set the priority on the switch to 24,576 (two increments lower than the default priority) while the root secondary command will set the priority on the secondary root to the 28,672 (one increment lower than the default priority).

  1. Modify D1 and D2 so that D1 is elected the primary root bridge for MST Instance 1 and D2 is elected the primary root bridge for MST Instance 2. D1 should be elected as the secondary root bridge for MST Instance 2, and D2 should be elected as the secondary root bridge for MST Instance 1. You will need to make configuration changes on both D1 and D2. The commands used at D1:

D1(config)# spanning-tree mst 1 root primary

D1(config)# spanning-tree mst 2 root secondary

  1. After you have configured both D1 and D2, go to A1 and issue the command show spanning-tree root. In this output, you will see the root bridges differentiated.

A1# show spanning-tree root

RootHello Max Fwd

MST InstanceRoot IDCostTimeAge DlyRoot Port

—————- ——————– ——— —– — —————

MST024576 d8b1.9028.af80022015Fa0/1

MST124577 d8b1.9028.af8020000022015Fa0/1

MST224578 d8b1.905d.c30020000022015Fa0/3

From the above output, you can see that the root port for VLAN 1 is F0/1 and the root port for VLAN 2 is F0/3. A1 is the root bridge for MST Instance 0 in this example.

Step 3:Adjust port cost values to impact root and designated port selection.

As the network is implemented right now, there are two direct paths between switch A1 and the root bridge for each MST. Path and port costs are evaluated to determine the shortest path to the root bridge. In the case where there are multiple equal cost paths to the root bridge, additional attributes must be evaluated. In our case, the lower interface number (for example, F0/1) is chosen as the Root Port, and the higher interface number (for example, F0/2) is put into a spanning tree Blocking state.

You can see which ports are blocked with the show spanning-tree vlan-id command or the show spanning-tree blockedports command. For now, examine VLAN 1 on D1.

  1. On A1, issue the commands show spanning-tree vlan 1 and show spanning-tree blockedports.

A1# show spanning-tree mst 1

##### MST1vlans mapped:2-3

Bridgeaddress f078.1647.4580priority32769 (32768 sysid 1)

Rootaddress d8b1.9028.af80priority24577 (24576 sysid 1)

portFa0/1cost200000rem hops 19

InterfaceRole Sts CostPrio.Nbr Type

—————- —- — ——— ——– ——————————–

Fa0/1Root FWD 200000128.1P2p

Fa0/2Altn BLK 200000128.2P2p

Fa0/3Altn BLK 200000128.3P2p

Fa0/4Altn BLK 200000128.4P2p

Fa0/23Desg FWD 2000000128.23P2p

A1# show spanning-tree blockedports

NameBlocked Interfaces List

——————– ————————————

MST0Fa0/2, Fa0/3, Fa0/4

MST1Fa0/2, Fa0/3, Fa0/4

MST2Fa0/1, Fa0/2, Fa0/4

Number of blocked ports (segments) in the system : 9

As you can see, MST 1 has its Root Port on F0/1 and F0/2, F0/3, and F0/4 are Alternate Blocking Ports.

To manipulate which port becomes the Root Port on non-root bridges, change the port cost (a value between 1 and 200,000,000) or port priority value (a value between 0 and 240 in increments of 16). Remember that this change could have an impact on downstream switches as well.

Note: The changes you are about to implement are considered topology changes and could have a significant impact on the overall structure of the spanning tree in your switch network. Do not make these changes in a production network without careful planning and prior coordination.

  1. On A1, shutdown interfaces F0/1 and F0/2, assign a new port cost of 1000 to F0/2 using the spanning-tree mst 1 cost value command, and then issue the no shutdown command on the ports.

A1# conf t

Enter configuration commands, one per line.End with CNTL/Z.

A1(config)# interface range f0/1-2

A1(config-if-range)# shutdown

A1(config-if-range)# exit

A1(config)# interface f0/2

A1(config-if)# spanning-tree mst 1 cost 1000

A1(config-if)# exit

A1(config)# interface range f0/1-2

A1(config-if-range)# no shutdown

A1(config-if-range)# exit

  1. Now verify that this impacts root port selection on A1 using the show spanning-tree vlan 1 and show spanning-tree blockedports commands.

A1# show spanning-tree mst 1

##### MST1vlans mapped:2-3

Bridgeaddress f078.1647.4580priority32769 (32768 sysid 1)

Rootaddress d8b1.9028.af80priority24577 (24576 sysid 1)

portFa0/2cost1000rem hops 19

InterfaceRole Sts CostPrio.Nbr Type

—————- —- — ——— ——– ——————————–

Fa0/1Altn BLK 200000128.1P2p

Fa0/2Root FWD 1000128.2P2p

Fa0/3Desg FWD 200000128.3P2p

Fa0/4Desg FWD 200000128.4P2p

Fa0/23Desg FWD 2000000128.23P2p

A1# show spanning-tree blockedports

NameBlocked Interfaces List

——————– ————————————

MST0Fa0/2, Fa0/3, Fa0/4

MST1Fa0/1

MST2Fa0/1, Fa0/2, Fa0/4

Number of blocked ports (segments) in the system : 7

From the output, you can see that the root port selected by A1 for VLAN 1 is now interface F0/2, and the port (and root) cost is now 1000.

Step 4:Adjust port priority values to impact root port selection.

The next method to impact root port selection is configured on the root bridge itself. In our current network topology, A1 has two connections to the root bridge for MST Instance 2, switch D2. The root port has been selected, in this case based on the lowest port ID. Port ID is made up of two values, labeled as Prio (Priority) and Nbr (Number).

Note: The port number is not necessarily equal to the interface ID. A switch may use any port number for STP purposes as long as they are unique for each port on the switch.

The port priority can be any value between 0 and 240, in increments of 16 (older switches may allow setting the priority in different increments).

  1. On A1, issue the command show spanning-tree mst 2 and take note of the port ID values listed.

A1# show spanning-tree mst 2

##### MST2vlans mapped:4-5

Bridgeaddress f078.1647.4580priority32770 (32768 sysid 2)

Rootaddress d8b1.905d.c300priority24578 (24576 sysid 2)

portFa0/3cost200000rem hops 19

InterfaceRole Sts CostPrio.Nbr Type

—————- —- — ——— ——– ——————————–

Fa0/1Altn BLK 200000128.1P2p

Fa0/2Altn BLK 200000128.2P2p

Fa0/3Root FWD 200000128.3P2p

Fa0/4Altn BLK 200000128.4P2p

Fa0/23Desg FWD 2000000128.23P2p

Close configuration window

As expected with two equal-cost paths to the root bridge, the lower port ID was selected as the root port.

  1. Modify the port priority of D2 interface G1/0/6 so that it becomes the preferred port by issuing the spanning-tree mst 2 port-priority value interface configuration command. Use a value of 64.

Open configuration window

D2# config t

Enter configuration commands, one per line.End with CNTL/Z.

D2(config)# interface range g1/0/5-6

D2(config-if-range)# shutdown

D2(config-if-range)# exit

D2(config)# interface g1/0/6

D2(config-if)# spanning-tree mst 2 port-priority 64

D2(config-if)# exit

D2(config)# interface range g1/0/5-6

D2(config-if-range)# no shutdown

D2(config-if-range)# exit

Close configuration window

  1. On A1, issue the show spanning-tree mst 2 command and you will see that Fa0/4 is now the selected root port. This selection is based on the lower priority value of D2 interface G1/0/6. Notice that the lower priority value does not appear in any A1 output.

Open configuration window

A1# show spanning-tree mst 2

##### MST2vlans mapped:4-5

Bridgeaddress f078.1647.4580priority32770 (32768 sysid 2)

Rootaddress d8b1.905d.c300priority24578 (24576 sysid 2)

portFa0/4cost200000rem hops 19

InterfaceRole Sts CostPrio.Nbr Type

—————- —- — ——— ——– ——————————–

Fa0/1Altn BLK 200000128.1P2p

Fa0/2Altn BLK 200000128.2P2p

Fa0/3Altn BLK 200000128.3P2p

Fa0/4Root FWD 200000128.4P2p

Fa0/23Desg FWD 2000000128.23P2p

Close configuration window

End of document

Device Configs – Final

Switch D1

D1# show run

Building configuration…

Current configuration : 9023 bytes

!

version 16.9

no service pad

service timestamps debug datetime msec

service timestamps log datetime msec

! Call-home is enabled by Smart-Licensing.

service call-home

no platform punt-keepalive disable-kernel-core

!

hostname D1

!

!

vrf definition Mgmt-vrf

!

address-family ipv4

exit-address-family

!

address-family ipv6

exit-address-family

!

no aaa new-model

switch 1 provision ws-c3650-24ts

!

login on-success log

!

crypto pki trustpoint SLA-TrustPoint

enrollment pkcs12

revocation-check crl

!

license boot level ipservicesk9

!

!

diagnostic bootup level minimal

!

spanning-tree mode mst

spanning-tree extend system-id

!

spanning-tree mst configuration

name CCNPv8

revision 2

instance 1 vlan 2-3

instance 2 vlan 4-5

!

spanning-tree mst 0-1 priority 24576

spanning-tree mst 2 priority 28672

!

!

redundancy

mode sso

!

transceiver type all

monitoring

!

class-map match-any system-cpp-police-topology-control

description Topology control

class-map match-any system-cpp-police-sw-forward

description Sw forwarding, L2 LVX data, LOGGING

class-map match-any system-cpp-default

description Inter FED, EWLC control, EWLC data

class-map match-any system-cpp-police-sys-data

description Learning cache ovfl, High Rate App, Exception, EGR Exception, NFL SAMPLED DATA, RPF Failed

class-map match-any system-cpp-police-punt-webauth

description Punt Webauth

class-map match-any system-cpp-police-l2lvx-control

description L2 LVX control packets

class-map match-any system-cpp-police-forus

description Forus Address resolution and Forus traffic

class-map match-any system-cpp-police-multicast-end-station

description MCAST END STATION

class-map match-any system-cpp-police-multicast

description Transit Traffic and MCAST Data

class-map match-any system-cpp-police-l2-control

description L2 control

class-map match-any system-cpp-police-dot1x-auth

description DOT1X Auth

class-map match-any system-cpp-police-data

description ICMP redirect, ICMP_GEN and BROADCAST

class-map match-any system-cpp-police-stackwise-virt-control

description Stackwise Virtual

class-map match-any non-client-nrt-class

class-map match-any system-cpp-police-routing-control

description Routing control and Low Latency

class-map match-any system-cpp-police-protocol-snooping

description Protocol snooping

class-map match-any system-cpp-police-dhcp-snooping

description DHCP snooping

class-map match-any system-cpp-police-system-critical

description System Critical and Gold Pkt

!

policy-map system-cpp-policy

!

!

interface GigabitEthernet0/0

vrf forwarding Mgmt-vrf

no ip address

shutdown

negotiation auto

!

interface GigabitEthernet1/0/1

switchport mode trunk

!

interface GigabitEthernet1/0/2

shutdown

!

interface GigabitEthernet1/0/3

shutdown

!

interface GigabitEthernet1/0/4

shutdown

!

interface GigabitEthernet1/0/5

switchport mode trunk

!

interface GigabitEthernet1/0/6

switchport mode trunk

!

interface GigabitEthernet1/0/7

shutdown

!

interface GigabitEthernet1/0/8

shutdown

!

interface GigabitEthernet1/0/9

shutdown

!

interface GigabitEthernet1/0/10

shutdown

!

interface GigabitEthernet1/0/11

shutdown

!

interface GigabitEthernet1/0/12

shutdown

!

interface GigabitEthernet1/0/13

shutdown

!

interface GigabitEthernet1/0/14

shutdown

!

interface GigabitEthernet1/0/15

shutdown

!

interface GigabitEthernet1/0/16

shutdown

!

interface GigabitEthernet1/0/17

shutdown

!

interface GigabitEthernet1/0/18

shutdown

!

interface GigabitEthernet1/0/19

shutdown

!

interface GigabitEthernet1/0/20

shutdown

!

interface GigabitEthernet1/0/21

shutdown

!

interface GigabitEthernet1/0/22

shutdown

!

interface GigabitEthernet1/0/23

shutdown

!

interface GigabitEthernet1/0/24

shutdown

!

interface GigabitEthernet1/1/1

shutdown

!

interface GigabitEthernet1/1/2

shutdown

!

interface GigabitEthernet1/1/3

shutdown

!

interface GigabitEthernet1/1/4

shutdown

!

interface Vlan1

no ip address

shutdown

!

ip forward-protocol nd

ip http server

ip http authentication local

ip http secure-server

!

!

control-plane

service-policy input system-cpp-policy

!

banner motd ^C D1, Multiple Spanning Tree ^C

!

line con 0

exec-timeout 0 0

logging synchronous

stopbits 1

line aux 0

stopbits 1

line vty 0 4

exec-timeout 0 0

privilege level 15

password cisco123

logging synchronous

login

line vty 5 15

login

!

!

end

Switch D2

D2# show run

Building configuration…

Current configuration : 8981 bytes

!

version 16.9

no service pad

service timestamps debug datetime msec

service timestamps log datetime msec

! Call-home is enabled by Smart-Licensing.

service call-home

no platform punt-keepalive disable-kernel-core

!

hostname D2

!

vrf definition Mgmt-vrf

!

address-family ipv4

exit-address-family

!

address-family ipv6

exit-address-family

!

no aaa new-model

switch 1 provision ws-c3650-24ts

!

login on-success log

!

license boot level ipservicesk9

!

diagnostic bootup level minimal

!

spanning-tree mode mst

spanning-tree extend system-id

!

spanning-tree mst configuration

name CCNPv8

revision 2

instance 1 vlan 2-3

instance 2 vlan 4-5

!

spanning-tree mst 1 priority 28672

spanning-tree mst 2 priority 24576

!

redundancy

mode sso

!

transceiver type all

monitoring

!

class-map match-any system-cpp-police-topology-control

description Topology control

class-map match-any system-cpp-police-sw-forward

description Sw forwarding, L2 LVX data, LOGGING

class-map match-any system-cpp-default

description Inter FED, EWLC control, EWLC data

class-map match-any system-cpp-police-sys-data

description Learning cache ovfl, High Rate App, Exception, EGR Exception, NFL SAMPLED DATA, RPF Failed

class-map match-any system-cpp-police-punt-webauth

description Punt Webauth

class-map match-any system-cpp-police-l2lvx-control

description L2 LVX control packets

class-map match-any system-cpp-police-forus

description Forus Address resolution and Forus traffic

class-map match-any system-cpp-police-multicast-end-station

description MCAST END STATION

class-map match-any system-cpp-police-multicast

description Transit Traffic and MCAST Data

class-map match-any system-cpp-police-l2-control

description L2 control

class-map match-any system-cpp-police-dot1x-auth

description DOT1X Auth

class-map match-any system-cpp-police-data

description ICMP redirect, ICMP_GEN and BROADCAST

class-map match-any system-cpp-police-stackwise-virt-control

description Stackwise Virtual

class-map match-any non-client-nrt-class

class-map match-any system-cpp-police-routing-control

description Routing control and Low Latency

class-map match-any system-cpp-police-protocol-snooping

description Protocol snooping

class-map match-any system-cpp-police-dhcp-snooping

description DHCP snooping

class-map match-any system-cpp-police-system-critical

description System Critical and Gold Pkt

!

policy-map system-cpp-policy

!

interface GigabitEthernet0/0

vrf forwarding Mgmt-vrf

no ip address

shutdown

negotiation auto

!

interface GigabitEthernet1/0/1

switchport mode trunk

!

interface GigabitEthernet1/0/2

shutdown

!

interface GigabitEthernet1/0/3

shutdown

!

interface GigabitEthernet1/0/4

shutdown

!

interface GigabitEthernet1/0/5

switchport mode trunk

!

interface GigabitEthernet1/0/6

switchport mode trunk

spanning-tree mst 2 port-priority 64

!

interface GigabitEthernet1/0/7

shutdown

!

interface GigabitEthernet1/0/8

shutdown

!

interface GigabitEthernet1/0/9

shutdown

!

interface GigabitEthernet1/0/10

shutdown

!

interface GigabitEthernet1/0/11

shutdown

!

interface GigabitEthernet1/0/12

shutdown

!

interface GigabitEthernet1/0/13

shutdown

!

interface GigabitEthernet1/0/14

shutdown

!

interface GigabitEthernet1/0/15

shutdown

!

interface GigabitEthernet1/0/16

shutdown

!

interface GigabitEthernet1/0/17

shutdown

!

interface GigabitEthernet1/0/18

shutdown

!

interface GigabitEthernet1/0/19

shutdown

!

interface GigabitEthernet1/0/20

shutdown

!

interface GigabitEthernet1/0/21

shutdown

!

interface GigabitEthernet1/0/22

shutdown

!

interface GigabitEthernet1/0/23

shutdown

!

interface GigabitEthernet1/0/24

shutdown

!

interface GigabitEthernet1/1/1

shutdown

!

interface GigabitEthernet1/1/2

shutdown

!

interface GigabitEthernet1/1/3

shutdown

!

interface GigabitEthernet1/1/4

shutdown

!

interface Vlan1

no ip address

shutdown

!

ip forward-protocol nd

ip http server

ip http authentication local

ip http secure-server

!

control-plane

service-policy input system-cpp-policy

!

banner motd ^C D2, Multiple Spanning Tree ^C

!

line con 0

exec-timeout 0 0

logging synchronous

stopbits 1

line aux 0

stopbits 1

line vty 0 4

login

line vty 5 15

login

end

Switch A1

A1# show run

Building configuration…

Current configuration : 2095 bytes

!

version 15.2

no service pad

service timestamps debug datetime msec

service timestamps log datetime msec

no service password-encryption

!

hostname A1

!

boot-start-marker

boot-end-marker

!

!

no aaa new-model

system mtu routing 1500

!

!

spanning-tree mode mst

spanning-tree extend system-id

!

spanning-tree mst configuration

name CCNPv8

revision 2

instance 1 vlan 2-3

instance 2 vlan 4-5

!

vlan internal allocation policy ascending

!

interface FastEthernet0/1

switchport mode trunk

!

interface FastEthernet0/2

switchport mode trunk

spanning-tree mst 1 cost 1000

!

interface FastEthernet0/3

switchport mode trunk

!

interface FastEthernet0/4

switchport mode trunk

!

interface FastEthernet0/5

switchport mode trunk

!

interface FastEthernet0/6

switchport mode trunk

!

interface FastEthernet0/7

switchport mode trunk

!

interface FastEthernet0/8

switchport mode trunk

!

interface FastEthernet0/9

switchport mode trunk

!

interface FastEthernet0/10

switchport mode trunk

!

interface FastEthernet0/11

switchport mode trunk

!

interface FastEthernet0/12

switchport mode trunk

!

interface FastEthernet0/13

switchport mode trunk

!

interface FastEthernet0/14

switchport mode trunk

!

interface FastEthernet0/15

switchport mode trunk

!

interface FastEthernet0/16

switchport mode trunk

!

interface FastEthernet0/17

switchport mode trunk

!

interface FastEthernet0/18

switchport mode trunk

!

interface FastEthernet0/19

switchport mode trunk

!

interface FastEthernet0/20

switchport mode trunk

!

interface FastEthernet0/21

switchport mode trunk

!

interface FastEthernet0/22

switchport mode trunk

!

interface FastEthernet0/23

switchport mode trunk

!

interface FastEthernet0/24

shutdown

!

interface GigabitEthernet0/1

switchport mode trunk

!

interface GigabitEthernet0/2

switchport mode trunk

!

interface Vlan1

!

ip http server

ip http secure-server

!

banner motd ^C A1, Multiple Spanning Tree ^C

!

line con 0

exec-timeout 0 0

logging synchronous

line vty 0 4

login

line vty 5 15

login

!

end

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