I have recently connected Catalyst 3550/48XF to the stack of 2 Catalyst 3548 using Gigastack GBIC.
Now port indicator on 3550 is flashing green very often (4-5 times per second) even though there are no any other connections to 3550 except 1000BASE-SX to another 3550 which is empty as well.
The corresponding port on 3548 flashing once in 1-2 seconds, 1000BASE-SX port flashing once in 1-2 seconds, but Gigastack port on 3550 flashing very often. There are no any additional configuration except the default one on any port of 3550, all services except HTTP are switched off, there is only one VLAN (VLAN1).
When I'm looking to traffic statistic on this port I can see no errors but too many dropped packets (around 10000).
I am wondering if this behavior could be due to spanning tree BPDUs flowing between the switches. You can try couple of things to get some hint about what is happening. Try isolating switches and see which particular connection triggers this behavior. You can check the status of your GBIC port and see whether it functions an access link or a trunk link. I think 3550s autonegotiate and form a trunk connection with the other end if possible, and if you have many vlans on the 3548 switches, probably BPDUs for all vlans are flowing to the 3550. You can also refer the release notes in this link and see if there are any known problems. If it not a production switch, using 'debug gigastack' may give you some clues as to what is happening.
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