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<td bgcolor="#336666"><b><font face="Arial" size="2" color="#FFFFFF">3</font></b><font face="Arial" size="2" color="#FFFFFF"><b>.1</b></font></td>
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<td bgcolor="#336666" width="100%"><strong>
<font face="Arial" size="2" color="#FFFFFF">Serial Point-to-Point Links</font></strong></td>
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<td bgcolor="#669999" height="25"><b>
<font face="Arial" size="2" color="#FFFFFF">3.1.7</font></b></td>
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<font face="Arial" size="2" color="#FFFFFF">Troubleshooting a serial
interface</font></strong></td>
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<font FACE="Arial" SIZE="2" COLOR="#000000">The output of the </font>
<font FACE="Courier New" SIZE="2" COLOR="#000000"><b>show interfaces
serial</b></font><font FACE="Arial" SIZE="2" COLOR="#000000"> command
displays information specific to serial interfaces. When HDLC is configured, �Encapsulation HDLC� should be reflected in the
output.
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"> When PPP
is configured, "Encapsulation PPP" should be seen in the output.
<img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12"></font><p><font face="Arial" size="2" COLOR="#000000">Five
possible problem states can be identified in the interface status line
of the </font><font FACE="Courier New" SIZE="2" COLOR="#000000"><b>
show interfaces serial</b></font><font face="Arial" size="2" COLOR="#000000">
display: <img border="0" src="../../images/3.gif" align="absmiddle" width="12" height="12"></font><ul>
<li><font face="Arial" size="2">Serial x is down, line protocol is
down</font></li>
<li><font face="Arial" size="2">Serial x is up, line protocol is
down</font></li>
<li><font face="Arial" size="2">Serial x is up, line protocol is up
(looped)</font></li>
<li><font face="Arial" size="2">Serial x is up, line protocol is
down (disabled)</font></li>
<li><font face="Arial" size="2">Serial x is administratively down,
line protocol is down</font></li>
</ul>
<p><font face="Arial" size="2">The </font>
<font size="2" face="Courier New"><b>show controllers</b></font><font face="Arial" size="2">
command is another important diagnostic tool when troubleshooting
serial lines. The </font><font size="2" face="Courier New"><b>show
controllers</b></font><font face="Arial" size="2"> output indicates
the state of the interface channels and whether a cable is attached
to the interface. In Figure
<img border="0" src="../../images/4.gif" align="absmiddle" width="12" height="12">, serial
interface 0/0 has a V.35 DTE cable attached. The command syntax
varies, depending on platform. For serial interfaces on Cisco 7000
series routers, use the </font><font size="2" face="Courier New"><b>
show controllers cbus</b></font><font face="Arial" size="2">
command.</font><p><font face="Arial" size="2">If the electrical
interface output is shown as UNKNOWN, instead of V.35, EIA/TIA-449,
or some other electrical interface type, an improperly connected
cable is the likely problem. A problem with the internal wiring of
the card is also possible. If the electrical interface is unknown,
the corresponding display for the </font>
<font size="2" face="Courier New"><b>show interfaces serial</b></font><font face="Arial" size="2">
<X> command will show that the interface and line protocol are down.</font><p>
<font face="Arial" size="2">Following are some debug commands that
are useful when troubleshooting serial and WAN problems:</font><ul>
<li><font size="2" face="Courier New"><b>debug serial interface</b></font><font face="Arial" size="2">
� Verifies whether HDLC keepalive packets are incrementing. If
they are not, a possible timing problem exists on the interface
card or in the network.
<img border="0" src="../../images/5.gif" align="absmiddle" width="12" height="12"></font></li>
<li><font size="2" face="Courier New"><b>debug arp</b></font><font face="Arial" size="2">
� Indicates whether the router is sending information about or
learning about routers (with ARP packets) on the other side of the
WAN cloud. Use this command when some nodes on a TCP/IP network
are responding, but others are not.</font></li>
<li><font size="2" face="Courier New"><b>debug frame-relay lmi</b></font><font face="Arial" size="2">
� Obtains Local Management Interface (LMI) information which is
useful for determining whether a Frame Relay switch and a router
are sending and receiving LMI packets. </font></li>
<li><font size="2" face="Courier New"><b>debug frame-relay events</b></font><font face="Arial" size="2">
� Determines whether exchanges are occurring between a router and
a Frame Relay switch.</font></li>
<li><font size="2" face="Courier New"><b>debug ppp negotiation</b></font><font face="Arial" size="2">
� Shows Point-to-Point Protocol (PPP) packets transmitted during
PPP startup where PPP options are negotiated.</font></li>
<li><font size="2" face="Courier New"><b>debug ppp packet</b></font><font face="Arial" size="2">
� Shows PPP packets being sent and received. This command displays
low-level packet dumps.</font></li>
<li><font size="2" face="Courier New"><b>debug ppp</b></font><font face="Arial" size="2">
� Shows PPP errors, such as illegal or malformed frames,
associated with PPP connection negotiation and operation.</font></li>
<li><font size="2" face="Courier New"><b>debug ppp authentication </b></font><font face="Arial" size="2">
� Shows PPP Challenge Handshake Authentication Protocol (CHAP) and
Password Authentication Protocol (PAP) packet exchanges.</font></li>
</ul>
<blockquote>
<p><font face="Arial" size="2"><b>Caution:</b> Debugging output is
assigned high priority in the CPU process and can render the
system unusable. For this reason, debug commands should only be
used to troubleshoot specific problems or during troubleshooting
sessions with Cisco technical support staff. It is good practice
to use debug commands during periods of low network traffic and
when the fewest users are online. Debugging during these periods
decreases the likelihood that increased debug command processing
overhead will affect system use.</font></p>
</blockquote>
<p>
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Lab Activity</b></a></font></p>
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<p>Lab Exercise: Troubleshooting a Serial Interface</p>
<p>In this lab, the students will configure a serial interface on the London and
Paris routers.</p>
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<p>e-Lab Activity: Troubleshooting a Serial Interface</p>
</font><font FACE="Arial" SIZE="2" COLOR="#000000">
<p><font color="#808080">In this lab, the student will configure a serial
interface on the London and Paris routers.</font></p>
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