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May 11 / Greg

Cisco – Troubleshoot POS Interface

Troubleshooting a POS(Packet Over Sonet) interface is quite different than your standard ethernet interface. The first thing you will obviously do is a:

show interface POS1/0/0

If you are down/down then you obviously aren’t seeing light across the link and you need to troubleshoot accordingly.

If you see the link as up/down, then you are having other issues. Either encapsulation or sonet issues. There is a starting article from Cisco that helps you trouble shoot up/down on POS interfaces. If you have had your link up and running for a while, then it is most likely not an encapsulation issue and more likely a sonet issue. This being the case you will need to do a little debugging.

terminal monitor !enables debug messages to your console
debug serial interface

You will most likely see:

000611: *Apr 26 21:49:04.146 CDT: POS1/0/0: HDLC myseq 12, mineseen 0, yourseen 0, line down
000612: *Apr 26 21:49:14.146 CDT: POS1/0/0: HDLC myseq 13, mineseen 0, yourseen 0, line down
000613: *Apr 26 21:49:24.146 CDT: POS1/0/0: HDLC myseq 14, mineseen 0, yourseen 0, line down

By default a sonet interface sends a keepalive every 10 seconds. Whenever you send a message, your myseq number will increase. The mineseen is the sequence number of the last acknowledged keepalive you sent to the neighbor. The yourseen value is the sequence number of the last received keepalive from a neighbor. As you can see above I’m sending and I’m not getting anything back. So what now?

Now we enable a local loopback on our pos interface.

interface POS1/0/0
loopback internal

This loops back to you. You should now see your debug output look something like the following:

000595: *Apr 26 21:45:24.138 CDT: POS1/0/0: HDLC myseq 53, mineseen 0*, yourseen 53, line down (looped)
000596: *Apr 26 21:45:34.138 CDT: POS1/0/0: HDLC myseq 54, mineseen 53*, yourseen 54, line down (looped)
000597: *Apr 26 21:45:44.138 CDT: POS1/0/0: HDLC myseq 55, mineseen 54*, yourseen 55, line down (looped)
000598: *Apr 26 21:45:54.138 CDT: POS1/0/0: HDLC myseq 56, mineseen 55*, yourseen 56, line down (looped)
000599: *Apr 26 21:46:04.138 CDT: POS1/0/0: HDLC myseq 57, mineseen 56*, yourseen 57, line down (looped)

Notice that the mineseen and the yourseen are starting to correctly increment. This proves that your interface is working properly. At this point, you can tell the provider than their equipment went boom boom.

If you want to loop it outward to the provider network you would issue:

interface POS1/0/0
loopback line

As always with debug commands, be cautious since these commands can consume large amounts of memory. Also be sure to disable debug when you are done. If you can’t remember which debugs you enabled, simply use the remove all command:

un all


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  1. Brad / May 16 2011

    Well, your problem in the first place, is that you tried to troubleshoot a POS

    interface. Last time i checked, anything POS is something you don’t want. 🙂

  2. Greg / May 17 2011

    Sonnet is still VERY popular…especially if you are in the carrier world. If you are an end user, you will most likely never see a sonet interface. 🙂

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