Some handy notes on Cisco IOS commands for Serial interface debugging

Router#debug serial interface

and

Router#debug serial event

and obviously...

Router#term mon

...if you're not on the console.

These will show the progress of the interface going up/down and the packet sequence between the router and the carriers NTE.

Some example debug messages you may see:

Jan 7 14:02:30: Serial0/1/0(out): StEnq, myseq 1, yourseen 0, DTE down

This line above represents the router sending a message to the NTE '...(out)...' and that the message is number 1 in the sequence '...myseq 1...' and that it has received zero messages back from the NTE '...yourseen 0...' and that it thinks the NTE is down '...DTE down'. The yourseen field will only increment when the router starts to receive packets from the carrier NTE.


Jan 7 14:24:29: Serial0/1/0(out): StEnq, myseq 209, yourseen 207, DTE up
Jan 7 14:24:29: Serial0/1/0(in): Status, myseq 209, pak size 14
Jan 7 14:24:39: Serial0/1/0(out): StEnq, myseq 210, yourseen 208, DTE up
Jan 7 14:24:39: Serial0/1/0(in): Status, myseq 210, pak size 14

The lines above show the normal output to expect when you enable debug serial event. You can see there is inbound and outbound traffic, and both the sent (myseq) and received (yourseen) packet counts are incrementing.

Finally, check out the following pages on the Cisco website for really detailed debugging info various serial interface types:

http://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1915.html

http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a758d.shtml