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## 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

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