Knowledge Base

Design & Configure

Multiple Spanning Tree Protocol (Cisco)

Technology: Network

Area: Switching

Vendor: Cisco

Title: MST – Multiple Spanning Tree Protocol (802.1s)

Software: 12.X , 15.X

Platform: Catalyst 2960, 3560, 3750, 3850, 4500, 6500

 

MST was developed to reduce the number of spanning tree instances to reasonable value thus reducing CPU cycles needed to calculate STP for each VLAN separately. Most networks have a few logical topologies which can be mapped to separate spanning tree. MST offers up to 16 instances where instance 0 (IST) serves special purpose for the rest 15 instances (MSTI). It is important to configure all switches with the same MST configuration to let them operate inside one single region. The default MST configuration for hello time(2s), forward delay(15s) and max age(20s) are the same like in STP/RSTP protocols.

 

To configure the single region MST use below commands:

SW2#configure terminal
SW2(config-mst)# spanning-tree mst configuration
SW2(config-mst)# instance 1 vlan 1-2000
SW2(config-mst)# instance 2 vlan 2001-4094
SW2(config-mst)# name Office1
SW2(config-mst)# revision 1
SW2(config-mst)# show


Pending MST configuration
Name      [office1]
Revision  1     Instances configured 3

 

Instance  Vlans mapped
--------  -----------------------------------------------------
0         none
1         1-2000
2         2001-4094
---------------------------------------------------------------

SW2(config-mst)# exit
SW2(config)# spanning-tree mst 0-2 priority 16384 -> SW2 will be the root for MST instances
SW2(config)# spanning-tree mode mst

Verification commands

To check the digest from MST configuration.

SW2# show spanning-tree mst configuration digest
% Name      [office1]
Revision  1     Instances configured 3
Digest          0xDDD722ACC557C99A947C6285022DEC87
Pre-std Digest  0x2D13C227F036BFCB1C9728D8F86F64AF

 

To check the MST state for all instance in region.

SW2# show spanning-tree mst
##### MST0    vlans mapped:   none
Bridge        address f0b2.e57d.a400  priority      16384 (16384 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
---------------- ---- --- --------- -------- --------------------------------
Gi1/0/1          Desg FWD 200000    128.1    P2p Edge
Gi1/0/21        Desg FWD 200000    128.21   P2p Bound(PVST) -> different STP protocol
Gi1/0/22        Desg FWD 200000    128.22   P2p Bound(PVST) -> different STP protocol
Gi1/0/23        Desg FWD 200000    128.23   P2p
Gi1/0/24        Desg FWD 200000    128.24   P2p

##### MST1    vlans mapped:   1-2000
Bridge        address f0b2.e57d.a400  priority      16385 (16384 sysid 1)
Root          this switch for MST1

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/0/1          Desg FWD 200000    128.1    P2p Edge
Gi1/0/21        Desg FWD 200000    128.21   P2p Bound(PVST) -> different STP protocol
Gi1/0/22        Desg FWD 200000    128.22   P2p Bound(PVST) -> different STP protocol
Gi1/0/23        Desg FWD 200000    128.23   P2p
Gi1/0/24         Desg FWD 200000    128.24   P2p

 

##### MST2    vlans mapped:  2000-4094
Bridge        address f0b2.e57d.a400  priority      16385 (16384 sysid 2)
Root          this switch for MST2

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/0/1          Desg FWD 200000    128.1    P2p Edge
Gi1/0/21        Desg FWD 200000    128.21   P2p Bound(PVST) -> different STP protocol
Gi1/0/22        Desg FWD 200000    128.22   P2p Bound(PVST) -> different STP protocol
Gi1/0/23        Desg FWD 200000    128.23   P2p
Gi1/0/24        Desg FWD 200000    128.24   P2p

 

SW1# show spanning-tree root
Root    Hello Max Fwd
MST Instance           Root ID          Cost    Time  Age Dly  Root Port
---------------- -------------------- --------- ----- --- ---  ------------
MST0             16384 f0b2.e57d.a400         0    2   20  15  Fa0/47
MST1             16385 f0b2.e57d.a400    200000    2   20  15  Fa0/47
MST2             16385 f0b2.e57d.a400    200000    2   20  15  Fa0/47

 

SW1# show spanning-tree mst
##### MST0    vlans mapped:   none
Bridge        address 001c.5895.9100  priority      32768 (32768 sysid 0)
Root          address f0b2.e57d.a400  priority      16384 (16384 sysid 0)
port    Fa0/47          path cost     0
Regional Root address f0b2.e57d.a400  priority      16384 (16384 sysid 0)
internal cost 200000    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
---------------- ---- --- --------- -------- --------------------------------
Fa0/1            Desg FWD 200000     32.3    P2p
Fa0/47          Root FWD 200000    128.51   P2p -> main forwarding link for MST IST
Fa0/48          Altn BLK 200000    128.52   P2p -> alternative forwarding link for MST IST

 

##### MST1    vlans mapped:   1-2000
Bridge        address 001c.5895.9100  priority      32769 (32768 sysid 1)
Root          address f0b2.e57d.a400  priority      16385 (16384 sysid 1)
port    Fa0/47          cost          200000    rem hops 19

 

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1            Desg FWD 200000     32.3    P2p
Fa0/47          Root FWD 200000    128.51   P2p -> main forwarding link for MST instance 1
Fa0/48          Altn BLK 200000    128.52   P2p -> alternative forwarding link for MST instance 1

 

##### MST2    vlans mapped:   1-2000
Bridge        address 001c.5895.9100  priority      32769 (32768 sysid 2)
Root          address f0b2.e57d.a400  priority      16385 (16384 sysid 2)
port    Fa0/47          cost          200000    rem hops 19


Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1            Desg FWD 200000     32.3    P2p
Fa0/47          Root FWD 200000    128.51   P2p -> main forwarding link for MST instance 2
Fa0/48          Altn BLK 200000    128.52   P2p -> alternative forwarding link for MST instance 2

Author: Marcin Bialy
 
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