# STP cost of Port channel

Apr 23rd, 2010

Hi all,

we have switch which has gigabit ports and 4 ports are bundled and all 4 are up in port channel

i know cost of 2gigbit is  3

but here we have 4 ports and cost of stp is 3?

can some one explain why this so

thanks

mahesh

`mahesh18 wrote:Hi Jonso it means any speed between 2Gbps to 9 gbps will have cost of 3 right?mahesh`

Mahesh

Good question. I suspect so although without a switch to test it may be that an 8Gbps etherchannel for example goes to 2 but i suspect not. Unfortunately don't have switches to test with.

But what we can say is that between 2Gbps and 9Gbps the cost shouldn't be anything other than 3 or 2.

Jon

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## Replies

Jon Marshall Fri, 04/23/2010 - 13:36
`mahesh18 wrote:Hi all,we have switch which has gigabit ports and 4 ports are bundled and all 4 are up in port channeli know cost of 2gigbit is  3but here we have 4 ports and cost of stp is 3?can some one explain why this sothanksmahesh     `

Mahesh

Because there are only so many values to use ie. 10Gbps = 2, 20Gbps = 1. So you are kind of limted with what values you can use, hence the reason 4Gbps and 2Gbps are both 3. Don't forget you can alter the cost if you want to influence which path traffic takes within your network.

Jon

mahesh18 Fri, 04/23/2010 - 13:50

Hi jon

if you can explanin me in more detail please

Port-channel: Po1

------------

Age of the Port-channel = 705d:19h:39m:50s

Logical slot/port = 14/10 Number of ports = 4

GC = 0x00000000 HotStandBy port = null

Port state = Port-channel Ag-Inuse

Protocol = -

Ports in the Port-channel:

Index Load Port EC state No of bits

------+------+------+------------------+-----------

0 11 Gi1/19 On 2

1 22 Gi1/20 On 2

2 44 Gi2/19 On 2

3 88 Gi2/20 On 2

sh spanning-tree int Po1

Vlan Role Sts Cost Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

VLAN0076 Desg FWD 3 128.1674 P2p

we know 1gb cost is 4 and here we have 4 interfaces with each bandwidth of 1GB and if we add them all it comes

to 16GB  i did not got how cost comes out to 3?

thanks

mahesh

Jon Marshall Fri, 04/23/2010 - 14:00
`mahesh18 wrote:we know 1gb cost is 4 and here we have 4 interfaces with each bandwidth of 1GB and if we add them all it comesto 16GB  i did not got how cost comes out to 3?thanksmahesh`

Mahesh

With STP costs -

1Gbps =  4

2Gbps =  3

10Gbps = 2

20Gbps = 1

So the faster the link the lower the cost. So 1Gbps  = 4 but 10Gbps does not mean 10 x 4 = 40 because 40 would be slower than 1Gbps.

Jon

mahesh18 Fri, 04/23/2010 - 14:14

Hi Jon

so it means any speed between 2Gbps to 9 gbps will have cost of 3 right?

mahesh

Jon Marshall Fri, 04/23/2010 - 14:17
`mahesh18 wrote:Hi Jonso it means any speed between 2Gbps to 9 gbps will have cost of 3 right?mahesh`

Mahesh

Good question. I suspect so although without a switch to test it may be that an 8Gbps etherchannel for example goes to 2 but i suspect not. Unfortunately don't have switches to test with.

But what we can say is that between 2Gbps and 9Gbps the cost shouldn't be anything other than 3 or 2.

Jon

mahesh18 Fri, 04/23/2010 - 14:26

Hi jon

i just checked on switch its cost is 3 with 8 port gig channel

so u were right.

regards

mahesh