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Loop back error and time issues...

 
 
Tim Fischer
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      10-16-2003
Today, for the first time since beginning our project to move to Cisco
equipment - we had a massive failure. Actually it was dealing with a single
port on our 3550.

Here is what I have on the problem:
At 11:10am no one on our Ethernet network (we have Token-ring as well
until conversion is complete), could talk to our mainframe. So I looked
into the logs of our 3550 switch. Checking the time on the switch - the
time was correct (11:15am).

The logs state though that on Oct 16th at "16:47:09
%ETHCNTR-3-LOOP_BACK_DETECTED: keepalice packet loop-back detected on
GigabitEthernet 0/11" occured. I tried relocating the cable and every port
shutdown the same way. Port 11 has the cable coming from another switch
which is connected to the Token-Ring router. This has been working for
quite some time without issue.

Anyone have any ideas? I searched Cisco and they say something about a
balun cable, but this cable is the same cable that has been plugged in for a
couple months.

Any ideas on the Time difference? What would cause logging to be off so
much?

Thanks in advance!

-Tim



 
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Steinar Haug
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      10-16-2003
["Tim Fischer"]

| Here is what I have on the problem:
| At 11:10am no one on our Ethernet network (we have Token-ring as well
| until conversion is complete), could talk to our mainframe. So I looked
| into the logs of our 3550 switch. Checking the time on the switch - the
| time was correct (11:15am).
|
| The logs state though that on Oct 16th at "16:47:09
| %ETHCNTR-3-LOOP_BACK_DETECTED: keepalice packet loop-back detected on
| GigabitEthernet 0/11" occured. I tried relocating the cable and every port
| shutdown the same way. Port 11 has the cable coming from another switch
| which is connected to the Token-Ring router. This has been working for
| quite some time without issue.
|
| Anyone have any ideas? I searched Cisco and they say something about a
| balun cable, but this cable is the same cable that has been plugged in for a
| couple months.

Cisco's explanation here is utterly useless.

The error message is caused by the keepalive packets that Cisco switches
send, by default at 10 second intervals. Here is what tcpdump shows for
such a keepalive packet (tcpdump -n 'ether[12:2]==0x9000'):

22:56:54.497159 0:b0:c2:89:60:4a 0:b0:c2:89:60:4a 9000 60:
0000 0100 0000 0000 0000 0000 0000 0000
0000 0000 0000 0000 0000 0000 0000 0000
0000 0000 0000 0000 0000 0000 0000

The packet has the same MAC address as source and desstination (this is
the MAC address of the switch port).

If this keepalive packet is seen on *input* to the same switchport that
sent it, the port shuts down, with the ETHCNTR-3-LOOP_BACK_DETECTED
message that you saw.

We haven't yet been able to determine the exact circumstances which
cause this to happen - but we have seen it enough times that we have
had to turn off loopback detection on all our 3550 switches. It is,
however, easy to imagine that this could be caused by spanning tree
problems. You can trigger it easily enough by connecting both ends
of a fiber to the same GBIC, creating en explicit loopback.

You can turn off the port shutdown with

no errdisable detect cause loopback

If you don't want to turn off loopback detection/shutdown, you should
definitely enable loopback recovery (off by default, requiring manual
reset, an *awful* default!):

errdisable recovery cause loopback
errdisable recovery interval 60

Note that this is only available in 12.1(13)EA1a and newer.

Steinar Haug, Nethelp consulting, http://www.velocityreviews.com/forums/(E-Mail Removed)
 
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Walter Roberson
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      10-16-2003
In article <(E-Mail Removed)>,
Tim Fischer <(E-Mail Removed)> wrote:
: At 11:10am no one on our Ethernet network (we have Token-ring as well
:until conversion is complete), could talk to our mainframe. So I looked
:into the logs of our 3550 switch. Checking the time on the switch - the
:time was correct (11:15am).

:The logs state though that on Oct 16th at "16:47:09
:%ETHCNTR-3-LOOP_BACK_DETECTED: keepalice packet loop-back detected on
:GigabitEthernet 0/11" occured.

Is it possible that 16:47:09 was the time since reboot, rather
than the absolute time? Do you have
service timestamps datetime localtime show-timezone
configured?
--
We don't need no side effect-ing
We don't need no scope control
No global variables for execution
Hey! Did you leave those args alone? -- decvax!utzoo!utcsrgv!roderick
 
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