Port path cost

Answered Question
Mar 27th, 2009
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Hi,


I am trying to determine why my port path costs on my switches are arround 3023 and not just 23 . Eg.


Port 1 (GigabitEthernet0/1) of VLAN0001 is forwarding

Port path cost 3004, Port priority 128, Port Identifier 128.1.

Designated port id is 128.129, designated path cost 19


So path cost is 3023.. But why is port cost 3004?. Where does the 30/3000 come from?


On another switch to the same bridge I get.


Port 1 (GigabitEthernet0/1) of VLAN0001 is forwarding

Port path cost 4, Port priority 128, Port Identifier 128.1.

Designated port id is 128.408, designated path cost 19


Cost 23 which is correct..


There is no spanning-tree cost configurations on the switch above or root bridge.


They are all using the short path cost method.


The only thing that I can find different is the following is enabled on the switch that gives 3023 cost:


no spanning-tree optimize bpdu transmission


Does any one know what this does?


Thanks

Correct Answer by Giuseppe Larosa about 8 years 1 month ago

Hello Caughan,


if the switch has spanning-tree uplinkfast enabled to make it unlikely to become the root:


spanning tree priority is increased to more the 49000


the STP costs of ports are incremented of 3000 each


Hope to help

Giuseppe


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Correct Answer
Giuseppe Larosa Fri, 03/27/2009 - 06:50
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Hello Caughan,


if the switch has spanning-tree uplinkfast enabled to make it unlikely to become the root:


spanning tree priority is increased to more the 49000


the STP costs of ports are incremented of 3000 each


Hope to help

Giuseppe


vaughancahill Fri, 03/27/2009 - 07:44
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Awesome! Wasn't aware uplinkfast did these changes. That explains it.


Phuc Le Wed, 01/08/2014 - 22:30
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Dear Giuseppe,


Can you please advise the difference between the following terms resulted from the command "sh spanning-tree detail" on an IOS switch:



1. cost of root path is 1.          (I think this is an accumative value to get to the Root Bridge)

2. designated path cost 0.      (I think this is a Cost advertised by the Root Bridge)



Can you please assist


Thank you


Regards

Phuc Le

chandra_rc16 Thu, 01/09/2014 - 00:12
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Hi Phuc Le,


Yes you're correct "Cost of root path" is nothing but the accumulated cost to reach the root bridge .


But the "Designated path cost" is the cost to reach it's designated bridge. For example, look at the below topology.

Here the root port between Sw1 & Sw2 is Fa0/4 port of Sw2. So the rooth path cost is 19+19=38

Here the designated port between Sw1 and Sw2 is Fa0/3 port of Sw1.. so the desingated port cost from Sw2 is nothing but the cost to reach the Fa0/3 of Sw1 i.e, 19.


Please rate if you find this helpful.


Regards,

Chandu

Phuc Le Mon, 01/13/2014 - 02:58
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Hi Chandu,


Thank you for your prompt reply.


So the "designated path cost"  between Sw0 and Sw1 is 0, which is the Cost advertised by its Designated Bridge, in this case being a Root Bridge?




Regards

Phuc Le

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