STP troubles

Hi Everyone,

I'm having problems with STP....still. It seems that most of the time I can figure out which ports will be forwarding and which will be blocking but as I'm adding more and more switches my predictions seem to be wrong.

I added in the switch highlighted (switch 0) and it has two possible paths back to the root, both with equal costs. So from looking at the MAC addresses of each switch I can see that:

Switch 5 - 000A is the lowest therefore this side will forward.
Switch 0 = 0003 and switch 6 = 0060 so Switch 0 has the lowest mac address and therefore my prediction was Switch 0 will forward and switch 6 will block but as you can see from the picture it is the other way around. Why is this?

STP.jpg

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Comments

mella060

Well switch 6 is closer to the root switch so i would imagine that it would forward the 'least cost bpdu' onto the segment between itself and switch 0. Correct me if i am wrong.

MrXpert

The more switches you add and in your case having more than 7 switches could invite problems. I think there is a recommended switch amount of 7 which is called the diameter. If you are using packet tracer then this has known issues with spanning tree, RSTP and portfast. I have seen some whacky results with STP on PT where every port was forwarding and sometimes the mac addresses change to random values every so often.

alliasneo

MrXpert wrote: »

The more switches you add and in your case having more than 7 switches could invite problems. I think there is a recommended switch amount of 7 which is called the diameter. If you are using packet tracer then this has known issues with spanning tree, RSTP and portfast. I have seen some whacky results with STP on PT where every port was forwarding and sometimes the mac addresses change to random values every so often.

Thats cool, that puts me at ease a little. My understanding with STP is that lowest wins so the higher switch will block but sometimes with Packet tracer it doesn't seem to do that so hopefully this is what the problem is.

thanks