The Fa0/1 was subsequently put into looped err-disabled state.įollowing this experiment, I have deactivated the LOOP protocol on the Fa0/1 and activated the STP instead:Īfter the Fa0/1 went up and started sending BPDUs, they again looped back. Upon receiving its own LOOP frame, the switch displayed on the console:Ġ0:34:01: %ETHCNTR-3-LOOP_BACK_DETECTED: Keepalive packet loop-back detected on FastEthernet0/1.Ġ0:34:01: %PM-4-ERR_DISABLE: loopback error detected on Fa0/1, putting Fa0/1 in err-disable stateĠ0:34:01: STP SW: Fa0/1 new disabled req for 1 vlansĠ0:34:02: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to downĠ0:34:03: %LINK-3-UPDOWN: Interface FastEthernet0/1, changed state to down The Fa0/1 started sending the LOOP frames that looped on the second switch and returned back. Then, I have activated the LOOP frames on the Fa0/1 using the commands: After this configuration, no frames were sent out the Fa0/1 interface. The STP, VTP, CDP and LOOP protocols were disabled by this cofniguration. The switch was booted with its configuration completely erased, and was initially configured using these commands: I watched for the frame reflection on the hub and observed the behavior of the switch under test. If the switch under test sent a frame via its Fa0/1 port, it arrived at the second switch, looped and returned back. The switch under test was connected via Fa0/1 to a hub (for sniffing purposes) and the hub was subsequently connected to another switch whose other two ports were intentionally connected together, creating a loop. I have tested the behavior of 2950, 29 series switches to a looped port. Please bear with me.įirst, the results of my lab experiment. I'm ready to change my mind again about this as I did several times in the past. I will look again at how you did your tests. We are speaking of something so basic, that received frames could be simply ignored and not passed to upper layers as suggested in above documents. It is reasonable that if the objective is to test capability to send and receive ethernet frames a test with a frame sent out on wire and then received back would be the most meaningful. This bug has been discussed in several threads in the forums. The fact that receving back its own keepalive frames is a sign of a loop for me is related to a famous bug on C3750 and other switches on fiber based ports where the suggested workaround was to disable keepalive on those ports. > If the LOOP frame was supposed to be received back, what would be the action if it was not received back? I've done some searches and most of links point to the following:ĮCTP packets can be sent to unicast, broadcast, or the ECTP reserved My first idea was that keepalive frames were sent out with MAC SA= router NIC MAC and MAC DA= broadcast. If with the port unplugged I disable keepalive (no keepalive under interface config) the ports comes up and it is even pingable (because when pinging an ethernet interface the packet is not sent out on the wire like it happens with serial or ATM interfaces).Įthernet interfaces have no carrier to be detected and no TDM framing to be detected. If I take an Rj-45 ethernet port for example an ethernet of a PA 4E and i leave it unplugged the interface is down/down Check it out.This is my current understanding of ethernet keepalive. I know this post is 3 -4 years old, but they had loopback testers then. I would get a new board that goes into a PCI slot or maybe a USB based ethernet device. More likely you have some damage in some chip which does not pay to even think about repairing. The only thing you could hope to replace would be the physical port but you should be able to see any damage. Most modern equipment everything is part of the main board of the machine. Pretty much after this you are stuck I suspect. If you were to get lucky you may have just bent one of the wires inside the port, you can take a flashlight and look to see if all the wires look straight. You have done the test that proves you have a issue with the ethernet port on the PC.ie you plug a different pc into the same cable and it works. This is actually used on data communication lines. There really is no such thing as a ethernet loopback plug.
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