Categories R&S

L2 technologies – Multiple Spanning-Tree Protocol – MST

I have never seen this in production but this is still part of the CCIE.

Let’s try to get a good view about multiple spanning-tree protocol.

 

L2 technologies – Multiple Spanning-Tree Protocol  – Network diagram

L2 technologies – Multiple Spanning-Tree Protocol - MST

 

L2 technologies – Multiple Spanning-Tree Protocol  – Configuration:

Following our network diagram, we will create 2 MST instances in the same region.

Vlan 10 and vlan 11 will be in instance 1.

Vlan 12 will be in instance 2.

SW10(config)#spanning-tree mst configuration
SW10(config-mst)#instance 1 vlan 10,11
SW10(config-mst)#instance 2 vlan 12
SW10(config-mst)#name REGION1
SW10(config-mst)#revision 1
SW10(config-mst)#show pending
Pending MST configuration
Name      [REGION1]
Revision  1     Instances configured 3

Instance  Vlans mapped
--------  ---------------------------------------------------------------------
0         1-9,13-4094
1         10-11
2         12

SW10(config-mst)#exit

SW10(config)#spanning-tree mode mst

 

We now repeat this configuration on all our switches.

 

Here is how the spanning-tree is now seen.

Instance 0 always refer to all the vlans and is there by default.

Instance 1 and instance 2 have been configured and running.

SW10#sh spanning-tree

MST0
  Spanning tree enabled protocol mstp
  Root ID    Priority    32768
             Address     aabb.cc00.0400
             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.cc00.0400
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et1/0               Desg FWD 2000000   128.5    Shr
Et1/1               Desg FWD 2000000   128.6    Shr
Po10                Desg FWD 1000000   128.65   Shr

MST1
  Spanning tree enabled protocol mstp
  Root ID    Priority    32769
             Address     aabb.cc00.0400
             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.cc00.0400
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et1/0               Desg FWD 2000000   128.5    Shr
Et1/1               Desg FWD 2000000   128.6    Shr
Po10                Desg FWD 1000000   128.65   Shr

MST2
  Spanning tree enabled protocol mstp
  Root ID    Priority    32770
             Address     aabb.cc00.0400
             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.cc00.0400
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et1/0               Desg FWD 2000000   128.5    Shr
Et1/1               Desg FWD 2000000   128.6    Shr
Po10                Desg FWD 1000000   128.65   Shr

 

L2 technologies – Multiple Spanning-Tree Protocol  – Root Bridge Election:

Now we can setup our root bridge.

SW10 will be the root bridge for instance 0 and 1, and SW11 the secondary

SW11 will be the root bridge for instance 2 and SW10 the secondary.

SW10(config)#spanning-tree mst 0 priority 0
SW10(config)#spanning-tree mst 1 priority 0
SW10(config)#spanning-tree mst 2 priority 4096

SW11(config)#spanning-tree mst 0 priority 4096
SW11(config)#spanning-tree mst 1 priority 4096
SW11(config)#spanning-tree mst 2 priority 0

 

Let’s verify how it looks on both of them.

SW10#sh spanning-tree root
                                        Root    Hello Max Fwd
MST Instance           Root ID          Cost    Time  Age Dly  Root Port
---------------- -------------------- --------- ----- --- ---  ------------
MST0                 0 aabb.cc00.0400         0    2   20  15
MST1                 1 aabb.cc00.0400         0    2   20  15
MST2                 2 aabb.cc00.0500   1000000    2   20  15  Po10

SW11#sh spanning-tree root
                                        Root    Hello Max Fwd
MST Instance           Root ID          Cost    Time  Age Dly  Root Port
---------------- -------------------- --------- ----- --- ---  ------------
MST0                 0 aabb.cc00.0400         0    2   20  15  Po10
MST1                 1 aabb.cc00.0400       100    2   20  15  Po10
MST2                 2 aabb.cc00.0500         0    2   20  15

 

Regarding MST Path Selection with Port Cost and Port Priority, it works the same than with normal spanning-tree.

 

 

Here is a great post about MSTP:

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

 

 

That’s all for multiple spanning-tree basics.

In the next post, I will build a new topology to play around with multiple spanning-tree on multiple region.

 

 

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

 

Leave a Reply

Your email address will not be published.