Out networks is built by 6506, 3750 and 2950 swiches. For some weeks ago all uplink ports in the 2950 switches went into error-disable caused by loopback-detected. The strange thing is that none of the 3750 swiches where affected even it the same vlans are wide spread all over the different type of devices. Does anyone know if there are some bugs in this 12.1(19) software that triggers loopback-detected???
A loopback error occurs when the keepalive packet is looped back to the port that sent the keepalive. The switch sends keepalives out all the interfaces by default. A device can loop the packets back to the source interface, which usually occurs because there is a logical loop in the network that the spanning tree has not blocked. The source interface receives the keepalive packet that it sent out, and the switch disables the interface (errdisable). This message occurs because the keepalive packet is looped back to the port that sent the keepalive:
%PM-4-ERR_DISABLE: loopback error detected on Gi4/1, putting Gi4/1 in
Keepalives are sent on all interfaces by default in Cisco IOS Software Release 12.1EA-based software. In Cisco IOS Software Release 12.2SE-based software and later, keepalives are not sent by default on fiber and uplink interfaces. For more information, refer to Cisco bug ID CSCea46385 ( registered customers only) .
The suggested workaround is to disable keepalives and upgrade to Cisco IOS Software Release 12.2SE or later.
I can't really understand the reason why cisco where turning of the keepalives in default from software releases above 12.2SE?
Is there any other feature that replaces the errdisable casue by loopback for uplink-ports or is are doesn't the keepalive packets belong on these type of interfaces where we have other spanning-tres mechanism that keeps the network free from loops?
I have seen this same issue occur on the network here. Uplink fiber trunk ports are err-disabled because of loopback and fastethernet ports are disabled because of loopback. So after checking the cables etc, i am very confident that there is no loop in the network. Can it be the use of old cable causing this errors to occur?
I have turned off keepalive on the uplink ports to at least keep the switches reachable in case of such events.
We are pleased to announce availability of Beta software for 16.6.3.
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