I was faced to a major issue on my LAN due to err_disable on several 2950s connected on two 6513.
After a problem on a GBIC on the 6513, we replaced it and when we switched on the interface,
all interfaces of 2650 connected on this 6513 were down due to err_disable.
I checked the syslog and err_disable was due to a keepalive packet loop-back.
I check on BugTool and there is a bug referenced about, CSCeg58877.
This bug seems to be applicable to our configuration.
I would like someone explains me why keepalive is implemented on interface and how it's work exactly.
I searched on cisco.com about "Loop Reply Receipts" but explanations are really poor.
Many thanks in advance.