How the Multi-Chassis role works

The switch performs the STP operation on the switch that has the Multi-Chassis role set to active. The switch with the role set to standby relays the information to the switch with the active role for STP tasks.

The primary VSX switch has the Multi-Chassis role set to active by default, just as the secondary VSX switch has the Multi-Chassis role set to standby by default. The Multi-Chassis role on the secondary VSX switch changes from standby to active if the primary VSX switch goes down.

The show spanning-tree vlan <VLAN-ID> detail command provides information about the value of the Multi-Chassis role. In the following example, the primary switch has the Multi-Chassis role set to active, and the secondary switch has the Multi-Chassis role set to standby.

Example of the Multi-Chassis role with the active value:

VSX-Primary# show spanning-tree vlan 2 detail

VLAN2
Spanning tree status : Enabled Protocol: RPVST
  Root ID    Priority   : 32768
             MAC-Address: 38:21:c7:66:24:00
             This bridge is the root
             Hello time(in seconds):2  Max Age(in seconds):20
             Forward Delay(in seconds):15

  Bridge ID  Priority  : 32768
             MAC-Address: 38:21:c7:66:24:00
             Hello time(in seconds):2  Max Age(in seconds):20
             Forward Delay(in seconds):15

Port         Role           State        Cost           Priority   Type
------------ -------------- ------------ ------------ ---------- ----------
1/3/1        Designated     Forwarding   1              128        point_to_point
lag2         Designated     Forwarding   20000          64         point_to_point

Topology change flag          : True
Number of topology changes    : 3
Last topology change occurred : 47 seconds ago

Port 1/3/1
Designated root has priority               :32768 Address: 38:21:c7:66:24:00
Designated bridge has priority             :32768 Address: 38:21:c7:66:24:00
Designated port                            : 1153
Number of transitions to forwarding state  : 1
Bpdus sent 28, received 28
TCN_Tx: 2, TCN_Rx: 0

Port lag2
Designated root has priority               :32768 Address: 38:21:c7:66:24:00
Designated bridge has priority             :32768 Address: 38:21:c7:66:24:00
Designated port                            : 770
Multi-Chassis role                         :active
Number of transitions to forwarding state  : 1
Bpdus sent 28, received 3
TCN_Tx: 2, TCN_Rx: 2

VSX-Secondary# show spanning-tree vlan 2 detail

VLAN2
Spanning tree status : Enabled Protocol: RPVST
  Root ID    Priority   : 32768
             MAC-Address: 38:21:c7:66:24:00
             This bridge is the root
             Hello time(in seconds):2  Max Age(in seconds):20
             Forward Delay(in seconds):15

  Bridge ID  Priority  : 32768
             MAC-Address: 38:21:c7:66:24:00
             Hello time(in seconds):2  Max Age(in seconds):20
             Forward Delay(in seconds):15

Port         Role           State        Cost           Priority   Type
------------ -------------- ------------ ------------ ---------- ----------
1/3/1        Designated     Forwarding   1              128        point_to_point
lag2         Designated     Forwarding   20000          64         point_to_point

Topology change flag          : False
Number of topology changes    : 2
Last topology change occurred : 35 seconds ago

Port 1/3/1
Designated root has priority               :32768 Address: 38:21:c7:66:24:00
Designated bridge has priority             :32768 Address: 38:21:c7:66:24:00
Designated port                            : 129
Number of transitions to forwarding state  : 2
Bpdus sent 24, received 22
TCN_Tx: 1, TCN_Rx: 2

Port lag2
Designated root has priority               :-32768 Address: 38:21:c7:66:24:00
Designated bridge has priority             :-32768 Address: 38:21:c7:66:24:00
Designated port                            :770
Multi-Chassis role                         :standby
Number of transitions to forwarding state  : 3
Bpdus sent 0, received 0
TCN_Tx: 0, TCN_Rx: 0