Multiple Spanning Tree
Topology
Objective
Obsrve te behavior of Multiple Spanning Tree(MST)
Background
Four switches have just been installed. The distribution Layer switches are Catalyst 3660s, and the access layer switches are Catalyst 2960. There are redundant uplinks between the access layer and distribution layer. Because of the possibility of bridging loops, spanning tree logically removes any redundant links. In this lab, we will group VLANs using MST so that we can have fewer spanning tree instances running at once to minimize switch CPU load.
Step 1: Prepare the switches for the lab.
- Delete vlan.dat file, erase the startup config, and reload the switches.
- Give each switch a hostname according to the topology diagram.
- Configure ports f0/7 through f0/12 on all switches to be trunks. On the 3560, first set the trunk encapsulation to dot1q. On the 2960s, only dot1q is supported, therefore the switchport trunk encapsulation command is unavailable, but the mode stll needs to be changed to trunk. If you do not set the mode of the ports to trunk, they will negotiate the operational mode according to their default DTP settings.
All Switches: interface range f0/7-12 switchport trunk encapsulation dot1q switchport mode trunk
Step 2: Configure VTP and VLANs
a. Configure all switches with VTP mode transparent and VTP domain CISCO. Add VLANs 10,20,30,40,50,60,70,80,90,100 to all of them.
All Switch: vtp mode transparent vtp domain CISCO vlan 10,20,30,40,50,60,70,80,90,100
b. Issue the show vlan brief command to view the VLAN configurations.
DLS1#show vlan brief
VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active Et1/2, Et1/3, Et2/0, Et2/1
Et2/2, Et2/3, Et3/0, Et3/1
Et3/2, Et3/3
10 VLAN0010 active
20 VLAN0020 active
30 VLAN0030 active
40 VLAN0040 active
50 VLAN0050 active
60 VLAN0060 active
70 VLAN0070 active
80 VLAN0080 active
90 VLAN0090 active
100 VLAN0100 active
1002 fddi-default act/unsup
1003 token-ring-default act/unsup
1004 fddinet-default act/unsup
1005 trnet-default act/unsup
Step 3: Display spanning-tree information
issue the show spanning-tree command on one of the switches. How many spanning tree instances are running?
DLS1#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address aabb.cc00.0100
Cost 100
Port 1 (Ethernet0/0)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address aabb.cc00.0500
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0 Root FWD 100 128.1 Shr
Et0/1 Altn BLK 100 128.2 Shr
Et0/2 Altn BLK 100 128.3 Shr
Et0/3 Altn BLK 100 128.4 Shr
Et1/0 Altn BLK 100 128.5 Shr
Et1/1 Altn BLK 100 128.6 Shr
Et1/2 Desg FWD 100 128.7 Shr
Et1/3 Desg FWD 100 128.8 Shr
Et2/0 Desg FWD 100 128.9 Shr
Et2/1 Desg FWD 100 128.10 Shr
Et2/2 Desg FWD 100 128.11 Shr
Et2/3 Desg FWD 100 128.12 Shr
Et3/0 Desg FWD 100 128.13 Shr
Et3/1 Desg FWD 100 128.14 Shr
Et3/2 Desg FWD 100 128.15 Shr
Et3/3 Desg FWD 100 128.16 Shr
VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 32778
Address aabb.cc00.0100
Cost 100
Port 1 (Ethernet0/0)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address aabb.cc00.0500
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0 Root FWD 100 128.1 Shr
Et0/1 Altn BLK 100 128.2 Shr
Et0/2 Altn BLK 100 128.3 Shr
Et0/3 Altn BLK 100 128.4 Shr
Et1/0 Altn BLK 100 128.5 Shr
Et1/1 Altn BLK 100 128.6 Shr
VLAN0020
Spanning tree enabled protocol ieee
Root ID Priority 32788
Address aabb.cc00.0100
Cost 100
Port 1 (Ethernet0/0)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32788 (priority 32768 sys-id-ext 20)
Address aabb.cc00.0500
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0 Root FWD 100 128.1 Shr
Et0/1 Altn BLK 100 128.2 Shr
Et0/2 Altn BLK 100 128.3 Shr
Et0/3 Altn BLK 100 128.4 Shr
Et1/0 Altn BLK 100 128.5 Shr
Et1/1 Altn BLK 100 128.6 Shr
VLAN0030
Spanning tree enabled protocol ieee
Root ID Priority 32798
Address aabb.cc00.0100
Cost 100
Port 1 (Ethernet0/0)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32798 (priority 32768 sys-id-ext 30)
Address aabb.cc00.0500
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0 Root FWD 100 128.1 Shr
Et0/1 Altn BLK 100 128.2 Shr
Et0/2 Altn BLK 100 128.3 Shr
Et0/3 Altn BLK 100 128.4 Shr
Et1/0 Altn BLK 100 128.5 Shr
Et1/1 Altn BLK 100 128.6 Shr
VLAN0040
Spanning tree enabled protocol ieee
Root ID Priority 32808
Address aabb.cc00.0100
Cost 100
Port 1 (Ethernet0/0)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32808 (priority 32768 sys-id-ext 40)
Address aabb.cc00.0500
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0 Root FWD 100 128.1 Shr
Et0/1 Altn BLK 100 128.2 Shr
Et0/2 Altn BLK 100 128.3 Shr
Et0/3 Altn BLK 100 128.4 Shr
Et1/0 Altn BLK 100 128.5 Shr
Et1/1 Altn BLK 100 128.6 Shr
VLAN0050
Spanning tree enabled protocol ieee
Root ID Priority 32818
Address aabb.cc00.0100
Cost 100
Port 1 (Ethernet0/0)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32818 (priority 32768 sys-id-ext 50)
Address aabb.cc00.0500
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0 Root FWD 100 128.1 Shr
Et0/1 Altn BLK 100 128.2 Shr
Et0/2 Altn BLK 100 128.3 Shr
Et0/3 Altn BLK 100 128.4 Shr
Et1/0 Altn BLK 100 128.5 Shr
Et1/1 Altn BLK 100 128.6 Shr
VLAN0060
Spanning tree enabled protocol ieee
Root ID Priority 32828
Address aabb.cc00.0100
Cost 100
Port 1 (Ethernet0/0)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32828 (priority 32768 sys-id-ext 60)
Address aabb.cc00.0500
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0 Root FWD 100 128.1 Shr
Et0/1 Altn BLK 100 128.2 Shr
Et0/2 Altn BLK 100 128.3 Shr
Et0/3 Altn BLK 100 128.4 Shr
Et1/0 Altn BLK 100 128.5 Shr
Et1/1 Altn BLK 100 128.6 Shr
VLAN0070
Spanning tree enabled protocol ieee
Root ID Priority 32838
Address aabb.cc00.0100
Cost 100
Port 1 (Ethernet0/0)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32838 (priority 32768 sys-id-ext 70)
Address aabb.cc00.0500
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0 Root FWD 100 128.1 Shr
Et0/1 Altn BLK 100 128.2 Shr
Et0/2 Altn BLK 100 128.3 Shr
Et0/3 Altn BLK 100 128.4 Shr
Et1/0 Altn BLK 100 128.5 Shr
Et1/1 Altn BLK 100 128.6 Shr
VLAN0080
Spanning tree enabled protocol ieee
Root ID Priority 32848
Address aabb.cc00.0100
Cost 100
Port 1 (Ethernet0/0)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32848 (priority 32768 sys-id-ext 80)
Address aabb.cc00.0500
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0 Root FWD 100 128.1 Shr
Et0/1 Altn BLK 100 128.2 Shr
Et0/2 Altn BLK 100 128.3 Shr
Et0/3 Altn BLK 100 128.4 Shr
Et1/0 Altn BLK 100 128.5 Shr
Et1/1 Altn BLK 100 128.6 Shr
VLAN0090
Spanning tree enabled protocol ieee
Root ID Priority 32858
Address aabb.cc00.0100
Cost 100
Port 1 (Ethernet0/0)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32858 (priority 32768 sys-id-ext 90)
Address aabb.cc00.0500
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0 Root FWD 100 128.1 Shr
Et0/1 Altn BLK 100 128.2 Shr
Et0/2 Altn BLK 100 128.3 Shr
Et0/3 Altn BLK 100 128.4 Shr
Et1/0 Altn BLK 100 128.5 Shr
Et1/1 Altn BLK 100 128.6 Shr
VLAN0100
Spanning tree enabled protocol ieee
Root ID Priority 32868
Address aabb.cc00.0100
Cost 100
Port 1 (Ethernet0/0)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32868 (priority 32768 sys-id-ext 100)
Address aabb.cc00.0500
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0 Root FWD 100 128.1 Shr
Et0/1 Altn BLK 100 128.2 Shr
Et0/2 Altn BLK 100 128.3 Shr
Et0/3 Altn BLK 100 128.4 Shr
Et1/0 Altn BLK 100 128.5 Shr
Et1/1 Altn BLK 100 128.6 Shr
Spanning tree is running a separate spanning tree instance for each VLAN created, plus VLAN1. This method assumes that each VLAN could be runnning on a differently shaped topology. However, in many networks, multiple VLAN follow the same physical topology, so multiple spanning-tree calculations for the same topologies can get redundant. MST lets you configure different spanning tree instances. Each instance can hold a group of VLANs and manages its own spanning tree calculation.
MST is convenient in that it is backward compatible with PVST and RPVST+. Two switches only run MST with each other if they are in the same MST region. An MST region is defined by switches having identical regioin names, revision numbers, and VLAN-to-instance assignments. If they differ by any single attribute, they are considered different MST regions and fall back to RPVST+.
Step 4: Configure MST globally.
a. To configure MST, first use the global configuration command spanning-tree mode mst on all four switches.
Note: By default, all VLANs are assigned to instance 0, but can be moved around to different instances when MST is configured.
b. issue the show spanning-tree command and observe that there is only one spanning tree(instance 0) coming up. Also notice that the mode is listed as MSTP.
DLS1(config)#do show spanning-tree
MST0
Spanning tree enabled protocol mstp
Root ID Priority 32768
Address aabb.cc00.0100
Cost 0
Port 1 (Ethernet0/0)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768 (priority 32768 sys-id-ext 0)
Address aabb.cc00.0500
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0 Root FWD 2000000 128.1 Shr
Et0/1 Altn BLK 2000000 128.2 Shr
Et0/2 Altn BLK 2000000 128.3 Shr
Et0/3 Altn BLK 2000000 128.4 Shr
Et1/0 Altn BLK 2000000 128.5 Shr
Et1/1 Altn BLK 2000000 128.6 Shr
Et1/2 Desg LRN 2000000 128.7 Shr
Et1/3 Desg LRN 2000000 128.8 Shr
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et2/0 Desg LRN 2000000 128.9 Shr
Et2/1 Desg FWD 2000000 128.10 Shr
Et2/2 Desg FWD 2000000 128.11 Shr
Et2/3 Desg FWD 2000000 128.12 Shr
Et3/0 Desg FWD 2000000 128.13 Shr
Et3/1 Desg FWD 2000000 128.14 Shr
Et3/2 Desg FWD 2000000 128.15 Shr
Et3/3 Desg FWD 2000000 128.16 Shr
c. if you use the show spanning-tree mst configuration command, you can see a switch‘s current MST configuration. Because you have not configured any MST region settings, the switch shows the default settings.
DLS1(config)#do show spanning-tree mst configuration
Name []
Revision 0 Instances configured 1
Instance Vlans mapped
-------- ---------------------------------------------------------------------
0 1-4094
-------------------------------------------------------------------------------
Step 5: Configure the MST region and instance.
Now that MST has been enabled, we can configure the MST region setting to group VLANs, We use the region name CISCO and revision number of 1. We put VLAN 20 through 50 into instance 1, and 80 and 100 into instance 2. The rest of the VLANs remain in instance 0, the default.
a. To begin modifying the MST configuration, type the global configuration command spanning-tree mst configuration. Configuring MST is deifferent from other switch configurations, because changes are not applied until you are finished (similar to the deprecated VLAN database mode), and you can abort changes if you wish.
Note: you must apply identical configurations on each switch for MST to work propely.
DLS1(config)#spanning-tree mst configuration DLS1(config-mst)#
b. When you are in MST configuration mode, you can view the current configuration using the show current command. You do not need to leave configuration mode to execute this command. Notice that the output is identical to show spanning-tree mst configuration.
change the region name by typing name name. Change the revision number by typing revison number.
DLS1(config)#spanning-tree mst configuration DLS1(config-mst)#name CISCO DLS1(config-mst)#revision 1
Note: The MST revision number is not like the configuration revision number used with VTP. It does not increment when changes are made. Along with the region name, the revisoin number identifies the MST domain and must be the same on all systems in the MST region.
c. The last configuration change you have to make is putting VLANs into instances. Use the command instance number vlan vlan_range. The instance number can be between 0 and 15. Remember that 0 is the default instance number.
DLS1(config)#spanning-tree mst configuration DLS1(config-mst)#name CISCO DLS1(config-mst)#revision 1 DLS1(config-mst)#instance 1 vlan 20-50DLS1(config-mst)#instance 2 vlan 80,100 DLS1(config-mst)#show current Current MST configuration Name [] Revision 0 Instances configured 1 Instance Vlans mapped -------- --------------------------------------------------------------------- 0 1-4094 -------------------------------------------------------------------------------
d. You can verify the changes you are about to make with show pending command. Remember that the changes that you just entered are not committed until you type exit, end or Ctrl + C, if you do not like the changes you made, you can leave the prompt without committing them by typing abort.
DLS1(config-mst)#show pending Pending MST configuration Name [CISCO] Revision 1 Instances configured 3 Instance Vlans mapped -------- --------------------------------------------------------------------- 0 1-19,51-79,81-99,101-4094 1 20-50 2 80,100 -------------------------------------------------------------------------------
Why do the switches wait until you are finished making changes to MST to commit them, rather than changing MST as you enter commands(like most switch commands)?
f. Verify that separate instances of spanning tree are running.
DLS2#show spanning-tree
MST0
Spanning tree enabled protocol mstp
Root ID Priority 32768
Address aabb.cc00.0100
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768 (priority 32768 sys-id-ext 0)
Address aabb.cc00.0100
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0 Desg FWD 2000000 128.1 Shr
Et0/1 Desg FWD 2000000 128.2 Shr
Et0/2 Desg FWD 2000000 128.3 Shr
Et0/3 Desg FWD 2000000 128.4 Shr
Et1/0 Desg FWD 2000000 128.5 Shr
Et1/1 Desg FWD 2000000 128.6 Shr
Et1/2 Desg FWD 2000000 128.7 Shr
Et1/3 Desg FWD 2000000 128.8 Shr
Et2/0 Desg FWD 2000000 128.9 Shr
Et2/1 Desg FWD 2000000 128.10 Shr
Et2/2 Desg FWD 2000000 128.11 Shr
Et2/3 Desg FWD 2000000 128.12 Shr
Et3/0 Desg FWD 2000000 128.13 Shr
Et3/1 Desg FWD 2000000 128.14 Shr
Et3/2 Desg FWD 2000000 128.15 Shr
Et3/3 Desg FWD 2000000 128.16 Shr
MST1
Spanning tree enabled protocol mstp
Root ID Priority 32769
Address aabb.cc00.0100
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address aabb.cc00.0100
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0 Desg FWD 2000000 128.1 Shr
Et0/1 Desg FWD 2000000 128.2 Shr
Et0/2 Desg FWD 2000000 128.3 Shr
Et2/0 Desg FWD 2000000 128.9 Shr
Et2/1 Desg FWD 2000000 128.10 Shr
Et2/2 Desg FWD 2000000 128.11 Shr
Et2/3 Desg FWD 2000000 128.12 Shr
MST2
Spanning tree enabled protocol mstp
Root ID Priority 32770
Address aabb.cc00.0100
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32770 (priority 32768 sys-id-ext 2)
Address aabb.cc00.0100
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0 Desg FWD 2000000 128.1 Shr
Et0/1 Desg FWD 2000000 128.2 Shr
Et0/2 Desg FWD 2000000 128.3 Shr
Et2/0 Desg FWD 2000000 128.9 Shr
Et2/1 Desg FWD 2000000 128.10 Shr
Et2/2 Desg FWD 2000000 128.11 Shr
Et2/3 Desg FWD 2000000 128.12 Shr
Challenge:
You can modify per-instance MST spanning tree attributes the same way you can modify per-VLAN attributes.
Make DLS1 the root of instance 1 and DLS2 the root of instance 2.
HINT: Use the question mark on the global configuration command spanning-tree mst ?
DLS1(config)#spanning-tree mst 1 root primary DLS2(config)#spanning-tree mst 2 root primary