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Community Member

setting spantree portcost

There's a descrepancy in the spantree portcost setting between the 2 config files on our cat 5000 taking within a 4 month period. To the best of my knowledge, no one manually changed them. Are they somehow getting auto reset depending on what is plugged in to the ports?


Cisco Employee

Re: setting spantree portcost

What are they getting changed to and from what? Also what software version are you running? Also what plugs to this port?

Community Member

Re: setting spantree portcost

Thanks for your reply.

What changed was the cost for the ports: for example, in the old config, the portcost of a port 6/1 was "19" and in the new config it was set to "100". The software version is 4.5 (13) and either a Netware 4.1 or NT 4.0 server were plugged into these ports.

Thanks again.


Re: setting spantree portcost

STP cost of 19 indicates a 100-megabit-per-second link speed, Fast Ethernet connection. Cost of 100 is for a 10-megabit-per-second link speed, Ethernet connection. These are Cisco's newer defaults; you can override these manually, if you want to precisely control how Spanning Tree sorts itself out, which is helpful in a mixed-manufacturer environment. (Not everybody costs their ports the same way.) But an all-Cisco network work fine with the defaults.

My guess is your port 6/1 is a 10/100 port set to autodetect speed and duplex.

Other STP costs in the Ethernet family: 4 is a 1-gigabit-per-second, Gigabit Ethernet connection; and I think 2 is for a 10-gigabit-per-second, 10 Gigabit Ethernet connection.

Cost generally used to be 1000 divided by link speed. This became a problem as speeds approached and then passed Gigabit, since the value passed along had to be an integer. (That is, you can't have a port cost of 0.4 for an OC-48 at 2.5-Gbps.) How would your network tell the difference between a Gig link and a 10-Gig link if they're both rounded to 1? Thus the recosting to the newer values.

Hope this helps.

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