calculate the cost of stp 10Gbps

[sumerfat]

Hi guys, I searched many times, still didn't find clear explanation, I asked ChatGpt, it gave a formula but the result 20 , still didn't answer how 2 came out from 20.

 

[Torbjørn]

802.1D-1998 and 802.1D-2004(both using 16-bit values) doesn't include an actual formula for the interface costs. Only a table of recommended values that Cisco switches adhere to. 

[sumerfat]

ok, but what are these 16 or 32 bits method? how the result of that 16 bits calculation is 20 , but they take the 2 answer.

 

[Torbjørn · Accepted solution]

The 16 bit refers to the cost value set for each interface, it can be a maximum of 65535(16 bit max value) and can differentiate interface speeds up to 10Gb(cost 2). All interfaces with speeds higher than 10G will be assigned a cost value of 1. You can find a list of the different cost values: https://www.hojmark.net/stp-port-cost. Since these values don't correspond to any general 802.1D cost formula you will simply have to memorize the values to be able to answer your quiz.

802.1t is an extension of 802.1D that defines (among other things) the 32 bit method. This uses a 32 bit cost value that allows for much more granular cost specification and uses "20Tbps/interface speed" to calculate the cost value. This allows for differentiation of interface speeds up to 20Tbps and is better suited for modern networks. This is the default method for spanning-tree cost calculation on newer Cisco switches.

You can read more about spanning tree and cost here: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9300/software/release/17-9/configuration_guide/lyr2/b_179_lyr2_9300_cg/configuring_spanning_tree_protocol.html

[balaji.bandi]

reference :

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