Configure the Spanning-Tree Path Cost

When a loop occurs in the network topology, STP uses path cost to determine the spanning-tree states of the ports. Path cost is obtained from the speed of the interface. You can configure a lower path cost for an interface if the port needs to be selected first, or you can configure a higher path cost if the port needs to be selected last to configure it to the forwarding state. The path cost is used to determine the topology only if the loop in the network cannot be resolved using only the bridge IDs. If all of the ports have the same path cost values, then the spanning tree puts the lowest numbered port into forwarding state.

See Topology Example for details on the topology that is referenced in the following procedure. After the topology stabilizes and switch A is elected as the root, the ports on all of the switches, except port 57 of switch C, should be in the forwarding state. Port 57 of switch C should be an alternate port and in the discarding state.

1. In switch C, specify the interface on which the path cost is to be configured.

   TOR-2 SEFOS# configure terminal
   TOR-2 SEFOS(config)# interface extreme-ethernet 0/1
   TOR-2 SEFOS(config-if)#

   Valid interfaces include physical interfaces and port-channel logical interfaces (port-channel port_channel_number).

2. Configure the cost for the interface (2000 in this example).

   TOR-2 SEFOS(config-if)# spanning-tree vlan 10 cost 2000
    
   Pvrst Vlan Cost is set  
    
   TOR-2 SEFOS(config-if)# end
   TOR-2 SEFOS#

   For cost, the range is 0 to 200000. The default value is derived from the media speed of the interface.

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   Note - After you configure the path cost for port 1 in switch C, you should observe that port 57 of switch B is the alternate port, and that port 57 of switch C is a designated port.

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3. In switch B, review the spanning tree information.

   TOR-1 SEFOS# show spanning-tree vlan 10
    
   -------------------------------------------------------------------
   Spanning-tree for VLAN 10
   Root Id         Priority   32778
                   Address    00:14:4f:6c:56:6f
                   Cost       2000
                   Port       Ex0/1
                   Hello Time 2 Sec, Max Age 20 Sec, Forward Delay 15 Sec
    
   Spanning Tree Enabled Protocol PVRST
   Bridge Id       Priority 32778
                   Address 00:14:4f:6c:69:ed
                   Hello Time 2 sec, Max Age 20 sec, Forward Delay 15 sec
   Name     Role         State        Cost     Prio   Type
   ----     ----         -----        ----     ----   ------
   Ex0/1    Root         Forwarding   2000     128    P2P
   Ex0/57   Alternate    Discarding   2000     128    P2P
   TOR-1 SEFOS#

4. In switch C, review the spanning tree information.

   TOR-2 SEFOS# show spanning-tree vlan 10
    
   -------------------------------------------------------------------
   Spanning-tree for VLAN 10
   Root Id         Priority   32778
                   Address    00:14:4f:6c:56:6f
                   Cost       200
                   Port       Ex0/1
                   Hello Time 2 Sec, Max Age 20 Sec, Forward Delay 15 Sec
    
   Spanning Tree Enabled Protocol PVRST
   Bridge Id       Priority 32778
                   Address 00:14:4f:6c:6e:0d
                   Hello Time 2 sec, Max Age 20 sec, Forward Delay 15 sec
   Name     Role         State        Cost     Prio   Type
   ----     ----         -----        ----     ----   ------
   Ex0/1    Root         Forwarding   200      128    P2P
   Ex0/57   Designated   Forwarding   2000     128    P2P
    
   TOR-2 SEFOS#

5. In switch C, reset the default value of the spanning-tree path cost.

   TOR-2 SEFOS# configure terminal
   TOR-2 SEFOS(config)# interface extreme-ethernet 0/1
   TOR-2 SEFOS(config-if)# no spanning-tree vlan 10 cost
   Pvrst Vlan Cost is set
   TOR-2 SEFOS(config-if)# end
   TOR-2 SEFOS#

6. In switch C, view the spanning tree information on VLAN 10.

   Port 57 should be in discarding state again.

   TOR-2 SEFOS# show spanning-tree vlan 10
    
   -------------------------------------------------------------------
   Spanning-tree for VLAN 10
   Root Id         Priority   32778
                   Address    00:14:4f:6c:56:6f
                   Cost       2000
                   Port       Ex0/1
                   Hello Time 2 Sec, Max Age 20 Sec, Forward Delay 15 Sec
    
   Spanning Tree Enabled Protocol PVRST
   Bridge Id       Priority 32778
                   Address 00:14:4f:6c:6e:0d
                   Hello Time 2 sec, Max Age 20 sec, Forward Delay 15 sec
   Name     Role         State        Cost     Prio   Type
   ----     ----         -----        ----     ----   ------
   Ex0/1    Root         Forwarding   2000     128    P2P
   Ex0/57   Alternate    Discarding   2000     128    P2P
    
   TOR-2 SEFOS#