Categories R&S

L2 technologies – MST multiple regions

Time to tackle some real MST challenge, multiple spanning-tree with multiple regions.

I have created a new network diagram in order to test this further.

 

L2 technologies – MST multiple regions – Physical network diagram

 

L2 technologies – MST multiple regions

 

L2 technologies – MST multiple regions – Theory:

Here is some theory first as MST with multiple region is not something that we use every day.

 

Every region will get its own IST (internal spanning-tree).

A regional root will be elected for each IST.

 

The common spanning tree (CST) is built across the region.

A common instance root will be elected for the complete instance

 

L2 technologies – MST multiple regions – configuring the regions:

Let’s first create the 3 regions.

At this time, we will use only one instance per region (the default one) and only vlan 1.

 

Here is the configuration for region A.

SW11(config)#spanning-tree mst configuration
SW11(config-mst)#name REGION-A
SW11(config-mst)#revision 1
SW11(config-mst)#exit
SW11(config)#spanning-tree mode mst

SW12(config)#spanning-tree mst configuration
SW12(config-mst)# name REGION-A
SW12(config-mst)# revision 1
SW12(config-mst)#exit
SW12(config)#spanning-tree mode mst

SW13(config)#spanning-tree mst configuration
SW13(config-mst)# name REGION-A
SW13(config-mst)# revision 1
SW13(config-mst)#exit
SW13(config)#spanning-tree mode mst

 

Now let’s do the same for the two other regions and see what the status of the spanning-after it has converge.

 

SW11 has been elected CIST root. All his port are designated and in forwarding state.

SW11#sh spanning-tree mst

##### MST0    vlans mapped:   1-4094
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
Et1/0            Desg FWD 2000000   128.5    Shr
Et1/1            Desg FWD 2000000   128.6    Shr

 

SW21 has been elected regional root.

Its port to SW11 (CIST root) has the root role and is in forwarding state, note that it shows type Shr Bound(RSTP).

Its port to its regional neighbor are designated and in forwarding state.

SW21#sh spanning-tree mst

##### MST0    vlans mapped:   1-4094
Bridge        address aabb.cc00.0400  priority      32768 (32768 sysid 0)
Root          address aabb.cc00.0100  priority      32768 (32768 sysid 0)
              port    Et1/0           path cost     2000000
Regional Root this switch
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
Et1/0            Root FWD 2000000   128.5    Shr Bound(RSTP)
Et1/1            Desg FWD 2000000   128.6    Shr

 

SW31 has been elected regional root.

Its port to SW11 (CIST root) has the root role and is in forwarding state, note that it shows type Shr Bound(RSTP).

Its port to SW12 is alternate and in blocking state and also shows type Shr Bound(RSTP).

Its port to its regional neighbor are designated and in forwarding state.

SW31#sh spanning-tree mst

##### MST0    vlans mapped:   1-4094
Bridge        address aabb.cc00.0700  priority      32768 (32768 sysid 0)
Root          address aabb.cc00.0100  priority      32768 (32768 sysid 0)
              port    Et1/1           path cost     2000000
Regional Root this switch
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
Et1/0            Altn BLK 2000000   128.5    Shr Bound(RSTP)
Et1/1            Root FWD 2000000   128.6    Shr Bound(RSTP)

 

L2 technologies – MST multiple regions – Root Bridge Election :

 

So what about the root bridge election when running MST with multiple regions?

The election goes this way:

  • First one regional root is elected per region
  • Then between all the regional roots a new election take place.

The election process is the same than usual for the spanning-tree.

 

Let’s say we want SW21 to become the CIST root.

SW21(config)#spanning-tree mst 0 priority 4096

 

The spanning-tree will recalculate and elect a new CIST root.

SW21#sh spanning-tree mst

##### MST0    vlans mapped:   1-4094
Bridge        address aabb.cc00.0400  priority      4096  (4096 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
Et1/0            Desg FWD 2000000   128.5    Shr
Et1/1            Desg FWD 2000000   128.6    Shr

 

L2 technologies – MST multiple regions and VTPv3:

With VTPv3, we should be able to create vlans on one switch and then propagate all the vlans in all the regions.

 

Let’s configure SW21 as VTPv3 primary server.

SW11 will be VTPv3 secondary server.

All the other switches will be VTPv3 client.

SW21(config)#vtp domain CISCO
SW21(config)#vtp version 3
SW21(config)#vtp mode server mst

SW21#vtp primary mst
This system is becoming primary server for feature  mst
%SW_VLAN-4-VTP_PRIMARY_SERVER_CHG: aabb.cc00.0400 has become the primary server for the MST VTP feature

 

If I tried to configure a vlan, I get the following:

SW21(config)#vlan 10
VTP VLAN configuration not allowed when device is not the primary server for vlan database.

 

So looks like I also need to make sure SW21 is the VTP primary server for feature vlan.

SW21#vtp primary
This system is becoming primary server for feature vlan
No conflicting VTP3 devices found.
Do you want to continue? [confirm]
SW21#
*Aug 13 20:19:51.080: %SW_VLAN-4-VTP_PRIMARY_SERVER_CHG: aabb.cc00.0400 has become the primary server for the VLAN VTP feature
SW21(config)#vlan 10
SW21(config-vlan)#end

 

And now the vlan 10 is propagated to all the switches:

SW13#sh vlan
VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
1    default                          active    Et0/2, Et0/3, Et1/0, Et1/1
                                                Et1/2, Et1/3
10   VLAN0010                         active

SW33#sh vlan
VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
1    default                          active    Et0/2, Et0/3, Et1/0, Et1/1
                                                Et1/2, Et1/3
10   VLAN0010                         active

 

I will also create vlan 20, 30 and 40.

 

L2 technologies – MST multiple regions – Configuring instances:

Let’s create 2 new instances on SW21.

 

Vlan 10 and 20 will be part of instance 1.

Vlan 30 and 40 will be part of instance 2.

SW21(config)#spanning-tree mst configuration
SW21(config-mst)#instance 1 vlan 10,20
SW21(config-mst)#instance 2 vlan 30,40

 

The instances have been replicated to all the other switches.

SW22#sh spanning-tree mst con
SW22#sh spanning-tree mst configuration
Name      [REGION-B]
Revision  1     Instances configured 3

Instance  Vlans mapped
--------  ---------------------------------------------------------------------
0         1-9,11-19,21-29,31-39,41-4094
1         10,20
2         30,40

SW11#sh spanning-tree mst configuration
Name      [REGION-B]
Revision  1     Instances configured 3

Instance  Vlans mapped
--------  ---------------------------------------------------------------------
0         1-9,11-19,21-29,31-39,41-4094
1         10,20
2         30,40

SW32#sh spanning-tree mst configuration
Name      [REGION-B]
Revision  1     Instances configured 3

Instance  Vlans mapped
--------  ---------------------------------------------------------------------
0         1-9,11-19,21-29,31-39,41-4094
1         10,20
2         30,40

 

Here is how our spanning-tree is looking after creating those 2 instances.

SW21#sh spanning-tree mst

##### MST0    vlans mapped:   1-9,11-19,21-29,31-39,41-4094
Bridge        address aabb.cc00.0400  priority      4096  (4096 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
Et1/0            Desg FWD 2000000   128.5    Shr
Et1/1            Desg FWD 2000000   128.6    Shr

##### MST1    vlans mapped:   10,20
Bridge        address aabb.cc00.0400  priority      32769 (32768 sysid 1)
Root          address aabb.cc00.0100  priority      32769 (32768 sysid 1)
              port    Et1/0           cost          2000000   rem hops 19

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Et0/0            Desg FWD 2000000   128.1    Shr
Et0/1            Desg FWD 2000000   128.2    Shr
Et1/0            Root FWD 2000000   128.5    Shr
Et1/1            Desg FWD 2000000   128.6    Shr

##### MST2    vlans mapped:   30,40
Bridge        address aabb.cc00.0400  priority      32770 (32768 sysid 2)
Root          address aabb.cc00.0100  priority      32770 (32768 sysid 2)
              port    Et1/0           cost          2000000   rem hops 19

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Et0/0            Desg FWD 2000000   128.1    Shr
Et0/1            Desg FWD 2000000   128.2    Shr
Et1/0            Root FWD 2000000   128.5    Shr
Et1/1            Desg FWD 2000000   128.6    Shr

 

After adding the instances we can see that SW21 is still the CIST for the instance 0.

But SW11 is the root for instance 1 and 2.

SW11#sh spanning-tree mst

##### MST0    vlans mapped:   1-9,11-19,21-29,31-39,41-4094
Bridge        address aabb.cc00.0100  priority      32768 (32768 sysid 0)
Root          address aabb.cc00.0400  priority      4096  (4096 sysid 0)
              port    Et1/0           path cost     0
Regional Root address aabb.cc00.0400  priority      4096  (4096 sysid 0)
                                      internal cost 2000000   rem hops 19
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
Et1/0            Root FWD 2000000   128.5    Shr
Et1/1            Desg FWD 2000000   128.6    Shr

##### MST1    vlans mapped:   10,20
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
Et1/0            Desg FWD 2000000   128.5    Shr
Et1/1            Desg FWD 2000000   128.6    Shr

##### MST2    vlans mapped:   30,40
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
Et1/0            Desg FWD 2000000   128.5    Shr
Et1/1            Desg FWD 2000000   128.6    Shr

 

Well this all for now on MST multiple regions.

 

I highly recommend to check this blog post on INE blog as this are the best explanations on MST that you will found.

http://blog.ine.com/2010/02/22/understanding-mstp/

 

 

 

Thank you for reading.

 

Have a look at my previous L2 technologies posts :

L2 technologies – VLANs & Trunking configuration

L2 technologies – VTP

L2 technologies – Etherchannel

L2 technologies – Spanning-tree Protocol

L2 technologies – Multiple Spanning-tree – MST

 

 

L2 technologies – MST multiple regions

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