for the third or fourth time in approx two years we have (painfully) noticed some really strange (at least to us) behaviour in our datacenters:
Uplink-interfaces on access-switches which have only one (!) physical uplink (lower end-points of the hierarchy) are going into err-disable state due to Loop-back detected.
There are no physical (wired) loops. How can this be?
RZ2-AS18# show version
Switch Ports Model SW Version SW Image
------ ----- ----- ---------- ----------
* 1 28 WS-C3560G-24TS 12.2(55)SE1 C3560-IPSERVICESK9-M
RZ2-AS18# show int des
Gi0/24 up up *** Uplink to rz2-cs4 Gi0/24 ***
RZ2-AS18# show logging
Jul 26 11:35:55.328: %ETHCNTR-3-LOOP_BACK_DETECTED: Loop-back detected on GigabitEthernet0/24.
Jul 26 11:35:55.328: %PM-4-ERR_DISABLE: loopback error detected on Gi0/24, putting Gi0/24 in err-disable state
Jul 26 11:35:56.611: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/24, changed state to down
Jul 26 11:35:57.651: %LINK-3-UPDOWN: Interface GigabitEthernet0/24, changed state to down
.Jul 26 12:12:04.173: %LINK-5-CHANGED: Interface GigabitEthernet0/24, changed state to administratively down
.Jul 26 12:12:06.295: %LINK-3-UPDOWN: Interface GigabitEthernet0/24, changed state to down
.Jul 26 12:12:09.131: %LINK-3-UPDOWN: Interface GigabitEthernet0/24, changed state to up
.Jul 26 12:12:10.137: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/24, changed state to up
.Jul 26 12:13:05.117: %SYS-5-CONFIG_I: Configured from console by console
rz2-as18# show run int gi 0/24
interface GigabitEthernet0/24
description *** rz2-cs4 Gi0/24 ***
switchport trunk encapsulation dot1q
switchport mode trunk
switchport nonegotiate
end
General network structure:
- ring between core-switches (CS)
- access-switches connected to core-switches in star-topology
Any ideas? Thoughts? Thanks in advance!
Best Answer
I think this might be related to a bug. This issue is documented in Cisco bug ID CSCea46385.
To resolve the issue, you need to add the no keepalive command to the interface.
Hope this helps you on your quest.
SleepyMan