Configure the Dynamic Path Cost in an Aggregate Port

The path cost of a port channel is the path cost corresponding to the aggregated speed of all the member ports of the port channel. The path cost of a port channel is determined when there is an operational status change for the port channel. There is no port channel path cost recalculation for operational status changes on individual member ports.

See Dynamic Path Cost Topology Example for the topology for this procedure. P1 and P2 are aggregated into a port channel interface.

1. View the spanning tree information in switch 2.

   SEFOS# show spanning-tree
    
   Root Id         Priority   32768
                   Address    00:01:02:03:04:01
                   Cost       200000
                   Port       1 [Ex0/1]
                  Max age 20 Sec, forward delay 15 Sec
                   Hello Time is 2 Sec
   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, forward delay is 15 sec
                   Hello Time is 2 sec                Dynamic Path Cost is Disabled                Dynamic Path Cost Lag-Speed Change is Disabled
   Name      Role         State        Cost     Prio   Type
   ----      ----         -----        ----     ----   ------
   Ex0/1     Root         Forwarding   200000   128    SharedLan
   Ex0/2     Alternate    Discarding   200000   128    SharedLan

2. Configure the path cost as dynamic in both switches.

   SEFOS# configure terminal
   SEFOS(config)# spanning-tree pathcost dynamic

3. Create and enable port channel in both switches.

   SEFOS(config)# set port-channel enable
   SEFOS(config)# interface port-channel 1
   SEFOS(config-if)# no shutdown
   SEFOS(config-if)# exit
   SEFOS(config)# interface extreme-ethernet 0/1
   SEFOS(config-if)# channel-group 1 mode active
   SEFOS(config-if)# exit
   SEFOS(config)# interface extreme-ethernet 0/2
   SEFOS(config-if)# channel-group 1 mode active
   SEFOS(config-if)# exit

4. View the spanning tree information in switch 2.

   SEFOS# show spanning-tree
    
   Root Id         Priority   32768
                   Address    00:01:02:03:04:01
                   Cost       99900
                   Port       25 [po1]
                   Max age 20 Sec, forward delay 15 Sec
                    Hello Time is 2 Sec
   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, forward delay is 15 sec
                   Hello Time is 2 sec                Dynamic Path Cost is Disabled                Dynamic Path Cost Lag-Speed Change is Disabled
   Name      Role         State        Cost     Prio   Type
   ----      ----         -----        ----     ----   ------
   po1       Root         Forwarding   99900    128    P2P

5. Disconnect the cable connecting port 2 of both the switches.
6. View the spanning tree information in switch 2.

   SEFOS# 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, forward delay 15 Sec
                    Hello Time is 2 Sec
   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, forward delay is 15 sec
                   Hello Time is 2 sec                Dynamic Path Cost is Disabled                Dynamic Path Cost Lag-Speed Change is Disabled
   Name      Role         State        Cost     Prio   Type
   ----      ----         -----        ----     ----   ------
   po1       Designated   Forwarding   99900    128    P2P

   ---

   Note - Even after the cable is removed, the path cost is not changed. The path cost will be recalculated only when there is an operational status change of the port channel interface, and not when the operational status of a port in a bundle changes.

   ---

7. In switch 2, change the administrative status of the port channel interface to down and then up.

   SEFOS(config)# interface port-channel 1
   SEFOS(config-if)# shutdown
   SEFOS(config-if)# no shutdown
   SEFOS(config-if)# end
   SEFOS# show spanning-tree
   Root Id         Priority   32768                Address    00:02:02:03:04:01                Cost       1900                Port       73 [po1]                Max age 20 Sec, forward delay 15 Sec                Hello Time 2 SecMST00Spanning tree Protocol has been enabledMST00 is executing the mstp compatible Multiple Spanning Tree ProtocolBridge Id       Priority  32768                Address  00:01:02:03:04:01                Max age is 20 sec, forward delay is 15 sec                Hello Time is 2 sec                Dynamic Path Cost is Disabled                Dynamic Path Cost Lag-Speed Change is EnabledName              Role         State        Cost     Prio   Type----              ----         -----        ----     ----   ------po1               Root         Forwarding   1900     128    P2P