Multiple Spanning Tree Protocol (802.1s)
Now that we have basic understanding of how MSTP works inside a region let’s create a sample configuration. Consider the following physical topology of three switches:
The topology has the following VLANs: 1, 10, 20, 30, 40, 50, 60. Our goals for this scenario are:
- Make VLANs 10,20,30 follow the link from SW3 to SW1.
- Make VLANs 40,50,60 follow the link from SW3 to SW2.
- If any of the above links fail, the affectred VLANs should fall-back to the other link.
To accomplish this, we create two MSTIs – number 1 and 2. SW1 will be the root for instance 1 and SW2 will be the root for instance 2. As for the IST (MSTI0), we make SW3 the root switch for it (though it’s not recommended to assign root roles to access switches). As for VLAN to MSTI mappings, VLAN 1 will remain mapped to the IST. Remaining VLANs 10, 20 and 30 would map MSTI1, while VLANs 40, 50 and 60 would map to MSTI2. Here is the configuration:
W1: spanning-tree mode mst ! spanning-tree mst configuration name REGION1 instance 1 vlan 10, 20, 30 instance 2 vlan 40, 50, 60 ! ! Root for MSTI1 ! spanning-tree mst 1 priority 8192 ! interface FastEthernet0/13 switchport trunk encapsulation dot1q switchport mode trunk ! interface FastEthernet0/16 switchport trunk encapsulation dot1q switchport mode trunk SW2: spanning-tree mode mst ! spanning-tree mst configuration name REGION1 instance 1 vlan 10, 20, 30 instance 2 vlan 40, 50, 60 ! ! Root for MSTI 2 ! spanning-tree mst 2 priority 8192 ! interface FastEthernet0/13 switchport trunk encapsulation dot1q switchport mode trunk ! interface FastEthernet0/16 switchport trunk encapsulation dot1q switchport mode trunk SW3: spanning-tree mode mst ! spanning-tree mst configuration name REGION1 instance 1 vlan 10, 20, 30 instance 2 vlan 40, 50, 60 ! ! Root for the IST ! spanning-tree mst 0 priority 8192 ! interface FastEthernet0/13 switchport trunk encapsulation dot1q switchport mode trunk ! interface FastEthernet0/16 switchport trunk encapsulation dot1q switchport mode trunk
The following show commands will demonstrate the effect our configuration has on traffic forwarding:
SW1#show spanning-tree mst configuration Name [REGION1] Revision 0 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 ------------------------------------------------------------------------------- SW1#show spanning-tree mst ##### MST0 vlans mapped: 1-9,11-19,21-29,31-39,41-49,51-59,61-4094 Bridge address 0019.5684.3700 priority 32768 (32768 sysid 0) Root address 0012.d939.3700 priority 8192 (8192 sysid 0) port Fa0/16 path cost 0 Regional Root address 0012.d939.3700 priority 8192 (8192 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 ---------------- ---- --- --------- -------- -------------------------------- Fa0/13 Desg FWD 200000 128.15 P2p Fa0/16 Root FWD 200000 128.18 P2p ##### MST1 vlans mapped: 10,20,30 Bridge address 0019.5684.3700 priority 8193 (8192 sysid 1) Root this switch for MST1 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Fa0/13 Desg FWD 200000 128.15 P2p Fa0/16 Desg FWD 200000 128.18 P2p ##### MST2 vlans mapped: 40,50,60 Bridge address 0019.5684.3700 priority 32770 (32768 sysid 2) Root address 001e.bdaa.ba80 priority 8194 (8192 sysid 2) port Fa0/13 cost 200000 rem hops 19 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Fa0/13 Root FWD 200000 128.15 P2p Fa0/16 Altn BLK 200000 128.18 P2p SW1#show spanning-tree mst interface fastEthernet 0/13 FastEthernet0/13 of MST0 is designated forwarding Edge port: no (default) port guard : none (default) Link type: point-to-point (auto) bpdu filter: disable (default) Boundary : internal bpdu guard : disable (default) Bpdus sent 561, received 544 Instance Role Sts Cost Prio.Nbr Vlans mapped -------- ---- --- --------- -------- ------------------------------- 0 Desg FWD 200000 128.15 1-9,11-19,21-29,31-39,41-49,51-59 61-4094 1 Desg FWD 200000 128.15 10,20,30 2 Root FWD 200000 128.15 40,50,60 SW1#show spanning-tree mst interface fastEthernet 0/16 FastEthernet0/16 of MST0 is root forwarding Edge port: no (default) port guard : none (default) Link type: point-to-point (auto) bpdu filter: disable (default) Boundary : internal bpdu guard : disable (default) Bpdus sent 550, received 1099 Instance Role Sts Cost Prio.Nbr Vlans mapped -------- ---- --- --------- -------- ------------------------------- 0 Root FWD 200000 128.18 1-9,11-19,21-29,31-39,41-49,51-59 61-4094 1 Desg FWD 200000 128.18 10,20,30 2 Altn BLK 200000 128.18 40,50,60
The link cost values are much higher than the default STP costs (IEEE standard values), and MSTIx is called MSTx (e.g. IST is MST0). Aside from that, note the term “Regional Root” which is to be explained in details below.
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