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amercer00
Contributor

2960 Receives it's own BPDU on the exact same port that it went out

We have a 2960.  All of the ports have portfast and BPDU Guard enabled.  Port 1 goes to a PC.  Port 2 goes to another PC.  Port 3 goes to port 1 on a hub.

We the take ports 2 and 3 on the hub and loop them into each other.  So the hub has a loop, but the switch doesn't.

I figured that the switch would see it's own BPDU on the port that it originally sent it on, and go err-disabled or BACKUP/Blocking.  However, it did not.  Layer 2 traffic started looping around on the hub and blasted back into our network through the uplink to the switch.

Any advice on why this happened or how to stop it.

 

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amercer00
Contributor

Quick update.  I replaced the hub with a little linksys switch (that doesn't support spanning tree itself).  The 2960 did err-disable Port 3 just like it was supposed to.

I also tried looping the hub back into the 2960 Port 4 and Port 4 went to Backup/Blocking like it should.