CSU and DSU Loopback Tests

If the output of the show interfaces serial exec command indicates that the serial line is up but the line protocol is down, use the CSU/DSU loopback tests to determine the source of the problem. Perform the local loop test first, and then the remote test. Figure 15-9 illustrates the basic topology of the CSU/DSU local and remote loopback tests.

Figure 15-9 CSU/DSU Local and Remote Loopback Tests

Local loop to Router A

Remote loop to Router B

Remote loop to Router A

Local loop to Router B

Router A

DSU/CSU

Figure 15-9 CSU/DSU Local and Remote Loopback Tests

Remote loop to Router B

DSU/CSU

DSU/CSU

DSU/CSU

Router B

Note These tests are generic in nature and assume attachment of the internetworking system to a CSU or DSU. However, the tests are essentially the same for attachment to a multiplexer with built-in CSU/DSU functionality. Because there is no concept of a loopback in X.25 or Frame Relay packet-switched network (PSN) environments, loopback tests do not apply to X.25 and Frame Relay networks.

CSU and DSU Local Loopback Tests for HDLC or PPP Links

Following is a general procedure for performing loopback tests in conjunction with built-in system diagnostic capabilities:

Step 1 Place the CSU/DSU in local loop mode (refer to your vendor documentation). In local loop mode, the use of the line clock (from the T1 service) is terminated, and the DSU is forced to use the local clock.

Step 2 Use the show interfaces serial exec command to determine whether the line status changes from "line protocol is down" to "line protocol is up (looped)," or if it remains down.

Step 3 If the line protocol comes up when the CSU or DSU is in local loopback mode, this suggests that the problem is occurring on the remote end of the serial connection. If the status line does not change state, there is a possible problem in the router, connecting cable, or CSU/DSU.

Step 4 If the problem appears to be local, use the debug serial interface privileged exec command.

Step 5 Take the CSU/DSU out of local loop mode. When the line protocol is down, the debug serial interface command output will indicate that keepalive counters are not incrementing.

Step 6 Place the CSU/DSU in local loop mode again. This should cause the keepalive packets to begin to increment. Specifically, the values for mineseen and yourseen keepalives will increment every 10 seconds. This information will appear in the debug serial interface output.

If the keepalives do not increment, there may be a timing problem on the interface card or on the network. For information on correcting timing problems, refer to the section "Troubleshooting Clocking Problems," earlier in this chapter.

Step 7 Check the local router and CSU/DSU hardware, and any attached cables. Make certain 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 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.

Figure 15-10 debug serial interface Command Output

1 missed keepalive

3 missed ^ keepalives

Figure 15-10 debug serial interface Command Output

router#	debug	serial interface
Seriall	HDLC	myseq	636ll9,	mineseen	636ll9,	yourseen	5l5032,	line	up
Seriall	HDLC	myseq	636l2 0,	mineseen	636l2 0,	yourseen	5l5033,	line	up
Seriall	HDLC	myseq	636l2l,	mineseen	636l2l,	yourseen	5l5034,	line	up
Seriall	HDLC	myseq	636l22,	mineseen	636l22,	yourseen	5l5035,	line	up
Seriall	HDLC	myseq	636l23,	mineseen	636l23,	yourseen	5l5036,	line	up
Seriall	HDLC	myseq	636l24,	mineseen	636l24,	yourseen	5l5037,	line	up
Seriall	HDLC	myseq	636l25,	mineseen	636l25,	yourseen	5l5038,	line	up
Seriall	HDLC	myseq	636l26,	mineseen	636l26,	yourseen	5l5 03 9,	line	up
Seriall	HDLC	myseq	636l27,	mineseen	636l27,	yourseen	5l5 04 0,	line	up
Seriall	HDLC	myseq	636l28,	mineseen	636l27,	yourseen	5l504l,	line	up
Seriall	HDLC	myseq	636l2 9,	mineseen	636l2 9,	yourseen	5l5042,	line	up
Seriall	HDLC	myseq	636l3 0,	mineseen	636l3 0,	yourseen	5l5043,	line	up
Seriall	HDLC	myseq	636l3l,	mineseen	636l3 0,	yourseen	5l5044,	line	up
Seriall	HDLC	myseq	636l32,	mineseen	636l3 0,	yourseen	5l5045,	line	up
Seriall	HDLC	myseq	636l33,	mineseen	636l3 0,	yourseen	5l5046,	line	down

Line goes down, interface - resets

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 xis 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.

Continue reading here: Modem State Modem Hardware State Meaning

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