Enabling Path Cost Re-calculation on Port Speed Change

The path cost of a port channel is the path cost corresponding to the aggregated speed of all member ports of the port channel.

The path cost of a port channel will be recalculated whenever a port becomes Active/In-Active in the channel group.

For setup, refer to Figure Topology for Configuring Dynamic Path Cost. P1 and P2 are aggregated into a port channel interface.

  1. Execute the following commands in the switch.

    Type the following:

    • View the output before configuring the dynamic path cost on speed change in Switch A.
    iS5comm# show spanning-tree
    Root Id         Priority   32768
    Address    00:01:02:03:04:01
    Cost       0
     Port      0 [0]
    This bridge is the root
     Max age 20 sec 0 cs, forward delay  15 sec 0 cs
    Hello Time 2 sec 0 cs
    MST00
    Spanning tree Protocol Enabled.
    MST00 is executing the mstp compatible Multiple Spanning Tree Protocol
    Bridge Id    Priority  32768
    Address  00:02:02:03:04:01
    Max age is 20 sec 0 cs, forward delay is 15 sec 0 cs
    Hello Time is 2 sec 0 cs
    Name    Role         State        Cost     Prio   Type
    ----    ----         -----        ----     ----   ------
    Gi0/1   Designated   Forwarding   20000    128    SharedLan
    Gi0/2   Designated   Discarding   20000    128    SharedLan

    Output before configuring the dynamic path cost on speed change in Switch B

    iS5comm# show spanning-tree
    Root Id         Priority   32768
    Address    00:01:02:03:04:01
    Cost       200000
     Port       1 [Gi0/1]
     Max age 20 sec 0 cs, forward delay  15 sec 0 cs
    Hello Time 2 sec 0 cs
    MST00
    Spanning tree Protocol Enabled.
    MST00 is executing the mstp compatible Multiple Spanning Tree Protocol
    Bridge Id    Priority  32768
    Address  00:02:02:03:04:01
    Max age is 20 sec 0 cs, forward delay is 15 sec 0 cs
    Hello Time is 2 sec 0 cs
    Name    Role         State        Cost     Prio   Type
    ----    ----         -----        ----     ----   ------
    Gi0/1   Root         Forwarding   20000    128    SharedLan
    Gi0/2   Alternate    Discarding   20000    128    SharedLan
  2. Configuration of dynamic cost calculation for Lagg

    Type the following:

    • Enter the Global Configuration Mode.
    iS5comm# configure terminal
    • Configure the dynamic path cost lag-speed.
    iS5comm(config)# spanning-tree pathcost dynamic lag-speed
    • Create and enable port channel in both the switches.
    iS5comm(config)# set port-channel enable
    iS5comm(config)# interface port-channel 1
    iS5comm(config-if)# no shutdown
    iS5comm(config-if)# exit
    iS5comm(config)# interface gigabitethernet 0/1
    iS5comm(config-if)# channel-group 1 mode on
    iS5comm(config-if)# exit
    • Return to the Privileged EXEC mode.
    iS5comm(config-if)#end
    • View the output in Switch B using the following command.
    iS5comm# show spanning-tre
     Root Id         Priority   32768
    Address    00:01:02:03:04:01
    Cost 99900
    Port 25 [po1] 
                  Max age 20 sec 0 cs, forward delay 15 sec 0 cs
                    Hello Time 2 sec 0 cs
    MST00
    Spanning tree Protocol Enabled.
    MST00 is executing the mstp compatible Multiple Spanning Tree Protocol
    Bridge Id   Priority  32768
                Address  00:02:02:03:04:01
                Max age is 20 sec 0 cs, forward delay is 15 sec 0 cs            Hello Time is 2 sec 0 cs
    Name    Role         State        Cost     Prio   Type
    ----    ----         -----        ----     ----   ------
    po1     Root         Forwarding   200000   128    P2P
  3. Make Port 2 as member of Port Channel on both switches.

    Type the following:

    iS5comm# configure terminal
    iS5comm(config)# interface gigabitethernet 0/2
    iS5comm(config-if)# channel-group 1 mode on
    iS5comm(config-if)# exit
  4. View the output in Switch B.

    Type the following:

    • View the default output in Switch B.
    iS5comm# show spanning-tree
    Root Id         Priority   32768
                    Address    00:01:02:03:04:01
                    Cost 100000
                    Port 25 [po1]
    This bridge is the root
    Max age 20 sec 0 cs, forward delay 15 sec 0 cs
    Hello Time 2 sec 0 cs
    

    MST00
    

    Spanning tree Protocol Enabled.
    

    MST00 is executing the mstp compatible Multiple Spanning Tree Protocol
    

    Bridge Id   Priority  32768
    

     Address  00:01:02:03:04:01
    

    Max age is 20 sec 0 cs, forward delay is 15 sec 0 cs
    

    Hello Time is 2 sec 0 cs
    

    Name    Role         State        Cost     Prio   Type
    

    ----    ----         -----        ----     ----   ------
    

    po1     Designated   Forwarding   100000   128    P2P
    

    5. 

  5. Remove Port 2 from Port Channel on both the switches.

    Type the following:
    

    iS5comm# configure terminal
    iS5comm(config)# interface gigabitethernet 0/2
    iS5comm(config-if)# no channel-group
    iS5comm(config-if)# exit
    6. 

  6. View the output in Switch B.

    Type the following:
    

    • View the default output
in Switch B.
      • 

    

    iS5comm# show spanning-tree
    Root Id         Priority   32768
    

                    Address    00:01:02:03:04:01
    

                    Cost 100000
    

                    Port 25 [po1]
    

    This bridge is the root
    

    Max age 20 sec 0 cs, forward delay 15 sec 0 cs
    

    Hello Time 2 sec 0 cs
    

    

    MST00
    

    Spanning tree Protocol Enabled.
    

    MST00 is executing the mstp compatible Multiple Spanning Tree Protocol
    

    Bridge Id   Priority  32768
    

     Address  00:01:02:03:04:01
    

    Max age is 20 sec 0 cs, forward delay is 15 sec 0 cs
    

    Hello Time is 2 sec 0 cs
    

    Name    Role         State        Cost     Prio   Type
    

    ----    ----         -----        ----     ----   ------
    

    Gi0/2   Alternate    Discarding   20000    128    SharedLan
    

    po1     Designated   Forwarding   20000    128    P2P
    

    7. 

  7. Execute the no spanning-tree pathcost dynamic lag-speed
Global Configuration command to disable dynamic pathcost calculation
on aggregated port speed change.

    Type the following:
    

    iS5comm(config)# no spanning-tree pathcost dynamic lag-speed
    Note: On
disabling dynamic pathcost calculation on speed change the calculated
pathcost value for Port channel will not be re-calculated. There
after the value will be recalculated for the port channel as the
Dynamic Path Cost calculation is explained in Section “Configuring
Dynamic Path cost in Aggregate Port”
    

    8. 









Parent topic: Configuring RSTP and MSTP