System information

Detailed Information on the show interfaces serial Command
Book Title
15-338
Figure 15-10 debug serial interface Command Output
CSU and DSU Remote Loopback Tests for HDLC or PPP Links
If you determine that the local hardware is functioning properly but you still encounter problems
when attempting to establish connections over the serial link, try using the remote loopback test to
isolate the problem cause.
Note This remote loopback test assumes that HDLC encapsulation is being used and that the
preceding local loop test was performed immediately before this test.
The following are the steps required to perform loopback testing:
Step 1 Put the remote CSU or DSU into remote loopback mode (refer to the vendor
documentation).
Step 2 Using the show interfaces serial exec command, determine whether the line protocol
remains up with the status line indicating “Serial x is up, line protocol is up (looped),” or
if it goes down with the status line indicating “line protocol is down.”
Step 3 If the line protocol remains up (looped), the problem is probably at the remote end of the
serial connection (between the remote CSU/DSU and the remote router). Perform both
local and remote tests at the remote end to isolate the problem source.
Step 4 If the line status changes to “line protocol is down” when remote loopback mode is
activated, make certain that ones density is being properly maintained. The CSU/DSU
must be configured to use the same framing and coding schemes used by the leased-line
or other carrier service (for example, ESF and B8ZS).
Step 5 If problems persist, contact your WAN network manager or the WAN service
organization.
Detailed Information on the show interfaces serial Command
This section covers the show interfaces serial command’s parameters, syntax description, sample
output display, and field descriptions.
router# debug serial interface
Serial1: HDLC myseq 636119, mineseen 636119, yourseen 515032, line up
Serial1: HDLC myseq 636120, mineseen 636120, yourseen 515033, line up
Serial1: HDLC myseq 636121, mineseen 636121, yourseen 515034, line up
Serial1: HDLC myseq 636122, mineseen 636122, yourseen 515035, line up
Serial1: HDLC myseq 636123, mineseen 636123, yourseen 515036, line up
Serial1: HDLC myseq 636124, mineseen 636124, yourseen 515037, line up
Serial1: HDLC myseq 636125, mineseen 636125, yourseen 515038, line up
Serial1: HDLC myseq 636126, mineseen 636126, yourseen 515039, line up
Serial1: HDLC myseq 636127, mineseen 636127, yourseen 515040, line up
Serial1: HDLC myseq 636128, mineseen 636127, yourseen 515041, line up
Serial1: HDLC myseq 636129, mineseen 636129, yourseen 515042, line up
Serial1: HDLC myseq 636130, mineseen 636130, yourseen 515043, line up
Serial1: HDLC myseq 636131, mineseen 636130, yourseen 515044, line up
Serial1: HDLC myseq 636132, mineseen 636130, yourseen 515045, line up
Serial1: HDLC myseq 636133, mineseen 636130, yourseen 515046, line down
1 missed
keepalive
3 missed
keepalives
Line goes
down,
interface
resets