PVST Lab Result

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

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Comments

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.

jrcarr2

In spanning-tree the cost is determined by the speed of the link, so as far as PVST is consicerned there is no difference in link.

When you change the priority on one port, you are actually changing the BID because BID = Priority + MAC address for the switch. This forces the decision on which port is designated.

It'd be better to use multiple switches.

volfkhat

jrcarr2 wrote: »

you are actually changing the BID because BID = Priority + MAC address for the switch.

Is that right?
Why isn't it:
BID = Priority + VLAN + MAC

?

CiscoWayne

volfkhat wrote: »

Is that right?
Why isn't it:
BID = Priority + VLAN + MAC

?

That's for PVST.

The original ST is just the priority+MAC

*Edit* I actually realised that this thread IS on about PVST, in which case, you're right

volfkhat

CiscoWayne wrote: »

That's for PVST.

The original ST is just the priority+MAC

*Edit* I actually realised that this thread IS on about PVST, in which case, you're right

Oh Wow..... i never noticed the Distinction.
Thank you for (unintentionally) pointing this out :]

TWX

That is a very important distinction...

I made a post on how that works. I don't think anyone read it but it's here if you want to:

www.techexams.net/forums/ccna-ccent/115074-stp-bridge-id-priority.html

EDIT: just copy/paste the URL, the site won't link right for whatever reason.