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STP root election error

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Hi,

I am setting up a lab exercise and came across this unusual problem when implementing the lab...

The lab is for a simple illustration of STP and its a simple setup with 3 switches: Switch1(3560), Switch2(3550) and Switch3(3560 PoE) connected in a full mesh that looks like a triangle..

I have not made any configurations, all switch ports are in access mode and am using cross-over cables to connect the switches to each other. The switch ports are auto-negotiating to 100/full.

The problem is: Switch2 has the lowest MAC and becomes the Root Bridge and this is acknowledged by Switch3. But Switch1 has a higher MAC than Switch2, and it still claims to be the Root Bridge. Because of this there is a loop and I get a flapping MAC address message.
I tried forcing the root priority values on Switch1 and Switch2, so as to make Switch2 the root, but that does not work and am having the same issue.
I also noticed that Switch1 was not receiving any BPDU messages, only sending them. Switch2 is reciveing and sending BPDU messages, and is correctly neglecting the Switch1 BPDU claims to be root.

I replaced Switch1 with a 4948 switch, and STP works correctly.
I don't know if the problem is with the physical switch itself or if there is some configuration on 3560 switches that I am unaware of.

Appreciate your replies.

Thanks.

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