STP Bandwidth Cost Formula ?

[inderdeeps]

Can anyone pl tell me the formula for calculating STP cost? How cost of 19 is arrived for 100Mbps interface & 4 is arrived for 1Gbps interface? Is there is any Formula or it is Pre-define values in the Switc ?

[Bilal Nawaz]

The costs are recommended by the IEEE in section 8.10.2  the 802.1D standard published in 1998 (available for free at the IEEE website).  However, the standard apparently makes no mention of the formula used  to arrive at these costs; if anyone can shed some light on this, please  leave a comment. The 1998 standard allows for a 16-bit path cost value  (held in software), 1 to 65535, and a 32-bit root path cost (the cost  which is advertised in a BPDU field).

Its successor, 802.1D-2004, increases the path cost to a 32-bit  value, providing far more granularity in assigning costs, and enabling  the use of a static scale.

http://standards.ieee.org/about/get/802/802.2.html

http://packetlife.net/blog/2008/sep/5/spanning-tree-port-costs/

Hope this helps

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[inderdeeps]

So you mean these are the Predefined values set by Cisco and have no formula to get it done manually.

Regards

Inderdeep

NETWORKS-BASELINE

http://www.routexp.com

[Bilal Nawaz · Accepted solution]

Predefined by the IEEE standards that cisco have implemented.

You can change the spanning-tree costs manually if you wanted to with the 'spanning-tree cost'

Please rate useful posts and remember to mark any solved questions as answered. Thank you.

[InayathUlla Sharieff · Accepted solution]

Hi Inder,

As of now I dont recollect the exact formula on the way the cost has been calculated to the bandwidth.

But you can consider higher the bandwidth lower will be the cost.

Yes there is a way to change the cost but we recommend not to play around with the same untill you have complete knowledge of the network/switches.

Please refer the below link on more info on STP cost:

http://en.wikipedia.org/wiki/Spanning_Tree_Protocol#Data_rate_and_STP_path_cost

http://www.cisco.com/en/US/customer/tech/tk389/tk621/technologies_tech_note09186a00800951ac.shtml

HTH

Regards

Inayath

[inderdeeps]

Thanks guys for this ...

Regards

Inderdeep

NETWORKS-BASELINE

http://www.routexp.com