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!