Multiple Spanning Tree Protocol

Multiple Spanning Tree Protocol - Cisco

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

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

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