What is MST multiple spanning tree IEEE 801s? What is the MSTP regions? What is MST instance? How to configure MSTP and tune or modify?

 IEEE 802.1s Multiple spanning tree (MST), sometimes referred to as multiple STP (MSTP). MST allows us to create Multiple spanning trees inside a single network. Each spanning tree is called an instance and this instance has its own root bridge, forwarding topology. An instance can be mapped with one or more VLANs. For example:

• VLANs 10,20 and 30 are mapped in instance 1
• VLANs 40,50 and 60 are mapped in instance 2

Now VLANs 10,20 and 30 follow different paths on the same physical network and VLANs 40,50 and 60 follow different paths and have different root bridges and forwarding paths.

multiple spanning tree is not like RPVST or PVST+ where each VLAN requires a separate instance. Let's take an example of PVST+ when we have 50 VLANs configured this means there are 50 instances of spanning tree running and this consumes a lot of CPU and memory. MST needs only one set of STP messages for each MST instance.  MST is the solution where we can map different VLANs to one or more instances as per the requirement.

 

An MST region is a group of switches that together use MST in a consistent way. They run the same number of MST instances and map the same sets of VLANs onto these instances, among other things. The switches in the MST region must have the same configuration parameters as follows:

 

• Region name: means the name of that particular region or group of MST switches.
• Revision number: means the number indicates the version of the configuration
• VLAN to Instance map: means a table of assignments for each VLAN to an instance

The switch outside the MST region will not see the MST region for outside the region switch, it looks like it's communicating to a big switch.

 

Multiple spanning tree (MST) instance

MST instance is a logical entity that defines a one-spanning tree for multiple VLANs, instance name is 32 bytes. MST instance number is in the range of 0 – 4095 but the different switch platforms support different numbers of range for example catalyst 2950 has the range of 0 – 15 and catalyst 3560 has the range of 0 – 4094. Now the important thing is out of the MST instance there is a special instance called instance 0 and this is also called the internal spanning tree (IST). Instance 0 or IST is default already exists. Instance 0 is the only instance that interacts with outside MST region switches and provides a loop-free environment to the region.  

Configuring MST requires pre-planning because it is necessary to decide where their MST boundaries shall be placed and where the regions shall be placed.  

MST allows us to tune the root bridge for instance modify costs and make primary or secondary.

 Now let's see the configuration:

Topology:

Goal:

• configure the trunk with nonnegotiable between switches ethernet 0/0-1
• configure VTP domain MSTP and version 2 for sync VLAN information
• configure switch 1 and switch 3 VTP server and the rest of the switch clients
• configure VLANs 10,20,30,40,50,60 on any VTP server
• configure Multiple spanning tree on all the switches 
• configure VLAN 10,20, and 30 in MST 1 instance 
• configure VLAN 40,50 and 60 in MST 2 instance
• configure region name MSTP and revision number 1
• by default our switch 1 is the root bridge for all the VLANs
• configure tunning for instance 1 means the VLAN 10,20,30 root bridge will be switch 1 and for instance, 2 switch 3 will be the root bridge. 

Switch-1-(config)#interface range ethernet 0/0 - 1
Switch-1-(config-if-range)#switchport
Switch-1-(config-if-range)#switchport trunk encapsulation dot1q
Switch-1-(config-if-range)#switchport mode trunk
Switch-1-(config-if-range)#switchport nonegotiate
Switch-1-(config-if-range)#exit

*Jun 12 10:05:37.389: %LINK-3-UPDOWN: Interface Ethernet0/0, changed state to up
*Jun 12 10:05:37.404: %LINK-3-UPDOWN: Interface Ethernet0/1, changed state to up

Switch-2-(config)#interface range ethernet 0/0 - 1
Switch-2-(config-if-range)#switchport
Switch-2-(config-if-range)#switchport trunk encapsulation dot1q
Switch-2-(config-if-range)#switchpot mode trunk
Switch-2-(config-if-range)#switchport nonegotiate
Switch-2-(config-if-range)#exit

*Jun 12 10:07:56.489: %LINK-3-UPDOWN: Interface Ethernet0/0, changed state to up
*Jun 12 10:07:56.521: %LINK-3-UPDOWN: Interface Ethernet0/1, changed state to up

Switch-3-(config)#interface range ethernet 0/0 - 1
Switch-3-(config-if-range)#switchport
Switch-3-(config-if-range)#switchport trunk encapsulation dot1q
Switch-3-(config-if-range)#switchport mode trunk
Switch-3-(config-if-range)#switchport nonegotiate
Switch-3-(config-if-range)#exit

*Jun 12 10:13:46.936: %LINK-3-UPDOWN: Interface Ethernet0/0, changed state to up
*Jun 12 10:13:46.951: %LINK-3-UPDOWN: Interface Ethernet0/1, changed state to up

Switch-4-(config)#interface range ethernet 0/0 - 1
Switch-4-(config-if-range)#switchport
Switch-4-(config-if-range)#switchport trunk encapsulation dot1q
Switch-4-(config-if-range)#switchport mode trunk
Switch-4-(config-if-range)#switch nonegotiate
Switch-4-(config-if-range)#exit
Switch-4-(config)#end

*Jun 12 10:16:12.123: %LINK-3-UPDOWN: Interface Ethernet0/0, changed state to up
*Jun 12 10:16:12.139: %LINK-3-UPDOWN: Interface Ethernet0/1, changed state to up

Switch-1-(config)#vtp domain MSTP

Switch-1-(config)#vtp version 2

Switch-1-(config)#vtp mode server

Switch-1-(config)#exit

Switch-1-#show vtp status

VTP Version capable             : 1 to 3

VTP version running             : 2

VTP Domain Name                 : MSTP

VTP Pruning Mode                : Disabled

VTP Traps Generation            : Disabled

Device ID                       : aabb.cc00.0100

Configuration last modified by 0.0.0.0 at 6-12-24 10:40:51

Local updater ID is 0.0.0.0 (no valid interface found)

Feature VLAN:

--------------

VTP Operating Mode                : Server

Maximum VLANs supported locally   : 1005

Number of existing VLANs          : 5

Configuration Revision            : 1

MD5 digest                        : 0x25 0xBA 0x4E 0x29 0x64 0xC6 0x11 0x76

                                    0xD4 0x1E 0xC2 0x6B 0x69 0xEB 0x26 0x7A

Switch-2-(config)#vtp domain MSTP

Switch-2-(config)#vtp version 2

Switch-2-(config)#vtp mode client

Switch-2-(config)#exit

Switch-3-(config)#vtp domain MSTP

Switch-3-(config)#vtp version 2

Switch-3-(config)#vtp mode client

Switch-3-(config)#exit

Switch-3-#show vtp status

VTP Version capable             : 1 to 3

VTP version running             : 2

VTP Domain Name                 : MSTP

VTP Pruning Mode                : Disabled

VTP Traps Generation            : Disabled

Device ID                       : aabb.cc00.0300

Configuration last modified by 0.0.0.0 at 6-12-24 10:40:51

Feature VLAN:

--------------

VTP Operating Mode                : Client

Maximum VLANs supported locally   : 1005

Number of existing VLANs          : 5

Configuration Revision            : 1

MD5 digest                        : 0x25 0xBA 0x4E 0x29 0x64 0xC6 0x11 0x76

                                    0xD4 0x1E 0xC2 0x6B 0x69 0xEB 0x26 0x7A

Switch-4-(config)#vtp domain MSTP

Switch-4-(config)#vtp version 2

Switch-4-(config)#vtp mode client

Switch-4-(config)#exit

Switch-1-(config)#vlan 10
Switch-1-(config-vlan)#exit
Switch-1-(config)#vlan 20
Switch-1-(config-vlan)#exit
Switch-1-(config)#vlan 30
Switch-1-(config-vlan)#exit
Switch-1-(config)#vlan 40
Switch-1-(config-vlan)#exit
Switch-1-(config)#vlan 50
Switch-1-(config-vlan)#exit
Switch-1-(config)#vlan 60
Switch-1-(config-vlan)#exit

Switch-2-#show vlan
VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
1    default                          active
10   VLAN0010                         active
20   VLAN0020                         active
30   VLAN0030                         active
40   VLAN0040                         active
50   VLAN0050                         active
60   VLAN0060                         active

Switch-3-#show vlan
VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
1    default                          active
10   VLAN0010                         active
20   VLAN0020                         active
30   VLAN0030                         active
40   VLAN0040                         active
50   VLAN0050                         active
60   VLAN0060                         active

(as you can see by default switch 1 is the root for all VLANs)

Switch-1-#show spanning-tree

VLAN0001

  Spanning tree enabled protocol ieee

  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

             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type

------------------- ---- --- --------- -------- --------------------------------

Et0/0               Desg BKN*100       128.1    Shr *PVID_Inc

Et0/1               Desg BKN*100       128.2    Shr *PVID_Inc

Switch-1-(config)#spanning-tree mode mst

Switch-1-(config)#spanning-tree mst configuration

Switch-1-(config-mst)#revision 1

Switch-1-(config-mst)#name MSTP

Switch-1-(config-mst)#instance 1 vlan 10,20,30

Switch-1-(config-mst)#instance 2 vlan 40,50,60

Switch-1-(config-mst)#exit

Switch-1-#show spanning-tree mst configuration

Name      [MSTP]

Revision  1     Instances configured 3

Instance  Vlans mapped

--------  ---------------------------------------------------------------------

0         1-9,11-19,21-29,31-39,41-49,51-59,61-4094

1         10,20,30

2         40,50,60

-------------------------------------------------------------------------------

Switch-1-#show spanning-tree mst

##### MST0    vlans mapped:   1-9,11-19,21-29,31-39,41-49,51-59,61-4094

Bridge        address aabb.cc00.0100  priority      32768 (32768 sysid 0)

Root          this switch for the CIST

Operational   hello time 2 , forward delay 15, max age 20, txholdcount 6

Configured    hello time 2 , forward delay 15, max age 20, max hops    20

Interface                        Role Sts Cost      Prio.Nbr Type

----------------                 ---- --- --------- -------- --------------------------------

Et0/0                            Desg FWD 2000000   128.1    Shr

Et0/1                            Desg FWD 2000000   128.2    Shr

##### MST1    vlans mapped:   10,20,30

Bridge        address aabb.cc00.0100  priority      32769 (32768 sysid 1)

Root          this switch for MST1

Interface                        Role Sts Cost      Prio.Nbr Type

----------------                 ---- --- --------- -------- --------------------------------

Et0/0                            Desg FWD 2000000   128.1    Shr

Et0/1                            Desg FWD 2000000   128.2    Shr

##### MST2    vlans mapped:   40,50,60

Bridge        address aabb.cc00.0100  priority      32770 (32768 sysid 2)

Root          this switch for MST2

Interface                        Role Sts Cost      Prio.Nbr Type

----------------                 ---- --- --------- -------- --------------------------------

Et0/0                            Desg FWD 2000000   128.1    Shr

Et0/1                            Desg FWD 2000000   128.2    Shr

Switch-2-(config)#spanning-tree mode mst

Switch-2-(config)#spanning-tree mst configuration

Switch-2-(config-mst)#revision 1

Switch-2-(config-mst)#name MSTP

Switch-2-(config-mst)#instance 1 vlan 10,20,30

Switch-2-(config-mst)#instance 2 vlan 40,50,60

Switch-2-(config-mst)#exit

Switch-3-(config)#spanning-tree mode mst

Switch-3-(config)#spanning-tree mst configuration

Switch-3-(config-mst)#revision 1

Switch-3-(config-mst)#name MSTP

Switch-3-(config-mst)#instance 1 vlan 10,20,30

Switch-3-(config-mst)#instance 2 vlan 40,50,60

Switch-3-(config-mst)#exit

Switch-4-(config)#spanning-tree mode mst

Switch-4-(config)#spanning-tree mst configuration

Switch-4-(config-mst)#revision 1

Switch-4-(config-mst)#name MSTP

Switch-4-(config-mst)#instance 1 vlan 10,20,30

Switch-4-(config-mst)#instance 2 vlan 40,50,60

Switch-4-(config-mst)#exit

Switch-1-#show spanning-tree mst 1

##### MST1    vlans mapped:   10,20,30

Bridge        address aabb.cc00.0100  priority      32769 (32768 sysid 1)

Root          this switch for MST1

Interface                        Role Sts Cost      Prio.Nbr Type

----------------                 ---- --- --------- -------- --------------------------------

Et0/0                            Desg FWD 2000000   128.1    Shr

Et0/1                            Desg FWD 2000000   128.2    Shr

Switch-1-#

Switch-1-#

Switch-1-#show spanning-tree mst 2

##### MST2    vlans mapped:   40,50,60

Bridge        address aabb.cc00.0100  priority      32770 (32768 sysid 2)

Root          this switch for MST2

Interface                        Role Sts Cost      Prio.Nbr Type

----------------                 ---- --- --------- -------- --------------------------------

Et0/0                            Desg FWD 2000000   128.1    Shr

Et0/1                            Desg FWD 2000000   128.2    Shr

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

Switch-1-(config)#spanning-tree mst 2 root secondary
Switch-1-(config)#exit

Switch-1-#show spanning-tree mst 1
##### MST1    vlans mapped:   10,20,30
Bridge        address aabb.cc00.0100  priority      24577 (24576 sysid 1)
Root          this switch for MST1
Interface                        Role Sts Cost      Prio.Nbr Type
----------------                 ---- --- --------- -------- --------------------------------
Et0/0                            Desg FWD 2000000   128.1    Shr
Et0/1                            Desg FWD 2000000   128.2    Shr

Switch-1-#show spanning-tree mst 2
##### MST2    vlans mapped:   40,50,60
Bridge        address aabb.cc00.0100  priority      28674 (28672 sysid 2)
Root          address aabb.cc00.0300  priority      2     (0 sysid 2)
            port    Et0/0           cost      4000000              rem hops 18
Interface                        Role Sts Cost      Prio.Nbr Type
----------------                 ---- --- --------- -------- --------------------------------
Et0/0                            Root FWD 2000000   128.1    Shr
Et0/1                            Altn BLK 2000000   128.2    Shr

(as you can see the above outcome switch 1 is the root bridge for MST instance 1 but not for MST instance 2)

Switch-3-(config)#spanning-tree mst 2 root primary

Switch-3-(config)#spanning-tree mst 1 root secondary

Switch-3-#show spanning-tree mst 2

##### MST2    vlans mapped:   40,50,60

Bridge        address aabb.cc00.0300  priority      2     (0 sysid 2)

Root          this switch for MST2

Interface                        Role Sts Cost      Prio.Nbr Type

----------------                 ---- --- --------- -------- --------------------------------

Et0/0                            Desg FWD 2000000   128.1    Shr

Et0/1                            Desg FWD 2000000   128.2    Shr

Switch-3-#show spanning-tree mst 1

##### MST1    vlans mapped:   10,20,30

Bridge        address aabb.cc00.0300  priority      28673 (28672 sysid 1)

Root          address aabb.cc00.0100  priority      24577 (24576 sysid 1)

            port    Et0/1           cost      4000000              rem hops 18

Interface                        Role Sts Cost      Prio.Nbr Type

----------------                 ---- --- --------- -------- --------------------------------

Et0/0                            Altn BLK 2000000   128.1    Shr

Et0/1                            Root FWD 2000000   128.2    Shr

(as you can see switch 3 is the root bridge for MST instance 2 but not for 1)

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