LAN issue

[danny dang]

                   Hi,

Some of our users intermittently got disconnected from the network and then reconnected back automatically after a few seconds. We are using Cisco IP phones and the desktop PC is connected to the Cisco IP phone. The issue seems to be switch related but we don't know how to confirm or how to get to the error messages on the switch. We are using Cisco Catalyst 3560 series switch. Any help please.

Thank you.

[harriet82]

you may want to check the router as well - are you saying the users are intermittently getting disconnected from the internet? or just internal lan?

[danny dang]

The main switch is WS-3560G-48PS. ..Thank you.

[Leo Laohoo]

The main switch is WS-3560G-48PS. ..Thank you.

Sorry, what do you mean "main switch"?

What I would like to know is what is the exact model of the switch(es) the PC/Phones are connected to.

If the switches are the same as above, then find a few of the PC+Phone that's having a problem and run the following commands:

1.  Command:  test cable tdr interface Gig 0/;

2.  Wait for approximately 8 seconds;

3.  Command:  sh cable tdr interface Gig 0/; and

4.  Post the output to #4.

[danny dang]

Hi,

Thanks for your response.

I meant "main switch" = core switch.

We rebooted the switch over the weekend and things seemed to be fine so far.

Below is the output on one of the port:

Interface Speed Local pair Pair length                 Remote pair    Pair status

---------      ----- ---------- ------------------ ----------- --------------------

Gi0/14    100M  Pair A     21   +/- 4  meters          Pair B           Normal

                        Pair B     44   +/- 4  meters         Pair A            Normal

                        Pair C     104  +/- 4  meters        Pair C            Fail

                        Pair D     151  +/- 4  meters        Pair D            Normal

These commands don't work on the trunking ports with SFF transceivers plugged in.

[Leo Laohoo]

Alright Danny.  Your output just confirmed my suspicion.  You have a definite cabling issue.

Pay close attention to Pair C and Pair D. 

Firstly, Pair "C" controls your PoE.  Notice the result?  Do you see it's stating that it's "Failed" instead of "Normal"?  Now look at the distance.  It's longer than the standard of 100 metres.

Next, Paid "D" controls GigabitEthernet.  Sure, the result is good that it's "Normal" but you WILL NOT get GigabitEthernet.  You know why?  Look at the distance.  Again, it's stating that it's >100 metres.

So aside from potential cabling issue, what does this tell me?  Look at the SIGNIFICANTLY large discrepancies between the cable distance of all four pairs.  It's between 21 to >100 metres.  I could go on-and-on here but what I'm suspecting you have is here is a sub-standard cabling from the patch panel (near your switch) to the remote end patch port (near your computer).   Another thing could be that the patch panel (near the computer), around 20 or so metres away, is causing the discrepancy.

How to use Time-Domain Reflectometer (TDR)

[danny dang]

It's just internal LAN problem.

[Leo Laohoo]

We are using Cisco Catalyst 3560 series switch. 

Could be a cabling issue.  What is the EXACT model of your switch(es)?