The switch reports this error message, and the port is forced to linkdown:
%ETHCNTR-3-LOOP_BACK_DETECTED : Keepalive packet loop-back detected on [chars]
This example shows the console output that is displayed when this problem occurs:
Oct 2 10:40:13: %ETHCNTR-3-LOOP_BACK_DETECTED: Keepalive packet loop-back detected on GigabitEthernet0/1
Oct 2 10:40:13: %PM-4-ERR_DISABLE: loopback error detected on Gi0/1, putting Gi0/1 in err-disable state
The problem occurs because the keepalive packet is looped back to the port that sent the keepalive. Keepalives are sent on the Catalyst switches in order to prevent loops in the network. Keepalives are enabled by default on all interfaces. You see this problem on the device that detects and breaks the loop, but not on the device that causes the loop.
Issue the no keepalive interface command in order to disable keepalives. A disablement of the keepalive prevents errdisablement of the interface, but it does not remove the loop.
Note: In Cisco IOS Software Release 12.2(x)SE-based releases and later, keepalives are not sent on fiber and uplink interfaces by default.
SRC and DEST MAC address is the same, address of interface
2. actually the idea behind was to check the avaiability of ethernet connection, Cisco uses keepalive in a different way - for loop detection.
3. if spanning tree convergence is not fast enough then keepalive can block some ports. (see my case). But usually STP converges much more faster then 10 sec (default for keepalive) and if some ports were err-disabled it usually means that STP configuration was wrong and it couldn't correctly block the links which built a loop.
4. it seems yes, because loopback detection reacts a little bit too hard on loopbacks.
We are pleased to announce availability of Beta software for 16.6.3.
16.6.3 will be the second rebuild on the 16.6 release train targeted
towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are
looking for early feedback from customers befor...
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