Information Technology Reference
In-Depth Information
Step 7
Check the local router and CSU/DSU hardware, and any attached cables. Make certain that the cables
are within the recommended lengths (no more than 50 feet [15.24 meters], or 25 feet [7.62 meters] for
a T1 link). Make certain that the cables are attached to the proper ports. Swap faulty equipment, as
necessary.
Figure 15-10 shows the output from the debug serial interface command for an HDLC serial
connection, with missed keepalives causing the line to go down and the interface to reset.
Figure15-10debug serial interface Command Output
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
1 missed
keepalive
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
Line goes
down,
interface
resets
3 missed
keepalives
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
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's cause.
This remote loopback test assumes that HDLC encapsulation is being used and that the
preceding local loop test was performed immediately before this test.
Note
The following are the steps required to perform loopback testing:
Put the remote CSU or DSU into remote loopback mode (refer to the vendor documentation).
Step 1
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 goes down, with the status line
indicating “line protocol is down.”
Step 2
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 3
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 4
If problems persist, contact your WAN network manager or the WAN service organization.
Step 5
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