Follow-up for Pre-study T216

The pre-studying for T216 has already been very useful. I had a question on Spanning Tree Protocol (STP) path costs. A scenario came up where the two ports of a network segment had different port speeds. I thought that logically, the two ports would auto-negotiate to the slower speed, and the cost of the path would be based on the new, lower port speed. The documentation I was reading, however, disagreed, and said that only the port speed of the root port mattered. So if it was higher than the designated port of that network segment, the cost would be lower and therefore more likely to be preferred when determining a blocking port further down the line.

It turns out that I was right, and the documentation was wrong. (At least for Cisco, which is what I need to know about.)  Further, somebody here was kind enough to not only quote proof, but attribute it for me, too:

On Cisco switches, the STP cost is based on the actual speed of the interface, so if an interface negotiates to use a lower speed, the default STP cost reflects that lower speed. If the interface negotiates to use a different speed, the switch dynamically changes the STP port cost as well.

-[CCNA ICND2 640-816 Official Cert Guide, Third Edition By: Wendell Odom] P.89 {Per-VLAN Port Costs}

Well that’s just about perfect.  I can actually use that at some point for T216, I’m sure, if they ask for an example of something something difficult something.

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