setting spantree portcost

[axfalk]

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?

Thanks.

[Prashanth Krishnappa]

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

[axfalk]

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.

[konigl]

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.