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Spanning tree Question=Cost


Can someone help me explain the output of sh spanning-tree int gi 1/2

I have an output here which is confusing

interface GigabitEthernet1/2
no switchport trunk native vlan tag
switchport private-vlan host-association 958 2958
switchport mode private-vlan host
no shutdown
no mab
snmp trap mac-notification change added
snmp trap mac-notification change removed
snmp trap link-status
no lldp transmit
no lldp receive
spanning-tree port-priority 3
spanning-tree cost 3

But in the output the cost shows as 4 but that should be 3 right 

sh spanning-tree int g1/2

Vlan                Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
VLAN0958            Desg FWD 4         128.1    P2p

This is on a 4500 switch

Peter Paluch
Hall of Fame Cisco Employee

Hi Chandu,

This is curious indeed. Neither the priority nor the cost seem to be influenced by the manual configuration on this switch. I wonder whether the output would change if you explicitly configured the cost 3 for VLANs 958 and 2958. Can you try this please? Also, can you post the output of show spanning-tree int g1/2 detail ?

Best regards,


Hey Peter

Thanx for replying back

Changing the cost on vlans did not make any difference as well

show spanning-tree int g1/2 det

Port 131 (FastEthernet3/3) of VLAN0282 is root forwarding

   Port path cost 19, Port priority 128, Port Identifier 128.131.

   Designated root has priority 8192, address 0010.7b55.9f09

   Designated bridge has priority 16384, address 00d0.635a.4809

   Designated port id is 128.1, designated path cost 4

   Timers: message age 2, forward delay 0, hold 0

   Number of transitions to forwarding state: 1

   Link type is point-to-point by default

   Loop guard is enabled by default on the port

   BPDU: sent 26, received 3959901

Other thing is i did not manually change the cost or priority  it was already there as 3 for both

Is it the default config, That does not happen with other switches

When i dug a little co worker copied sh run all from prev sup and copied to this when they replaced it few months ago

I think this is how it was but we just noticed  ,, luks weird