PVST Lab Result

squawk1500

I'm messing around with pvst and rapid-pvst using a single 3750 switch and got a result that I didn't expect and I'm trying to figure it out. If anyone can help I'd appreciate it!

I connect a cable from f1/0/1 to f1/0/3.
Same switch, same BID = tie
Same cost, 19 = tie
Same priority, 128 = tie
Internal interface, f1/0/1 vs f1/0/3 = f1/0/1 lowest
Result = f1/0/1 becomes designated, f1/0/3 becomes blocked
Kinda makes sense.

Below is were I start to run into problems because I change the cost of f1/0/3 to 4 and it doesn't become the designated port. I even used the speed 10 cmd on f1/0/1.

I connect a cable from f1/0/1 to f1/0/3, again...lol
Same switch, same BID = tie
Different cost, f1/0/1 = 19 but f1/0/3 = 4
Same priority, 128 = tie
Internal interface, f1/0/1 vs f1/0/3 = f1/0/1 lowest
Result = f1/0/1 becomes designated, f1/0/3 becomes blocked. But why if cost is lower???

I noticed if I changed the priority on f1/0/3 from 128 to 112 the ports switched.

So...I don't get it. I used my other 3750 and when cost was changed the ports changed too, des to blocking. The only thing I can think of is it's something to do with the fact that when you are using a single switch and it's connected to itself like that it's not going to believe me when I tell it that the cost is different on one side of the cable vs the other. idk

*spelling, oops

TWX

I don't think that it's a good idea to use a single switch to do spanning-tree experimenting to expect to learn the behavior. Even if all you pick up is a ghetto-old 2900XL or 3500XL to play with you'll probably do better.