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178 CHAPTER 12: CONFIGURING PPP AND MP
[Router]interface serial2: 1
[Router-Serial2:1]ppp mp
[Router-Serial2:1]ppp authentication-mode pap
[Router-Serial2:1]ppp pap local-user router-c password simple
router-c
Fault Diagnosis and
Troubleshooting of
PPP
Fault 1: Link always fails to turn to up status.
Troubleshooting: It is possible that PPP authentication parameter is not configured
correctly, resulting in the failure of PPP authentication.
Turn on the debugging switch of PPP, if LCP negotiation is successful and turns to
Up status, then begin PAP or CHAP negotiation and LCP turns to Down status.
Fault 2: Physical link fails to turn to Up status.
Troubleshooting: Execute display interface serial interface-number
command to view the current interface status, including five status:
serial number is administratively down, line protocol is down
Indicates that the interface is shutdown.
serial number is down, line protocol is down
Indicates that the interface is not activated or the physical layer does not turn to
Up status.
serial number is up, line protocol is up(spoofing)
Indicates that this interface is a dialup interface and the call is not connected
successfully.
serial number is up, line protocol is up
Indicates that data can be transmitted through this interface.
serial number is up, line protocol is down
Indicates that this interface is activated, but link negotiation is not successful.
Fault 3: Fail to ping through the peer although the link is UP and LCP and
IPCP are all opened.
Troubleshooting:
■ Execute the display running-configcurrent-configuration interface
command on the local end to check whether the IP address is configured and
whether IP address negotiation is configured with the
ip address
ppp-negotiate
command.
■ Check the configuration of the peer to see whether it assigns IP address to the
local end with the
remote address command.
■ Directly specify IP address for the local end or assign IP address to the peer.
Then reset the interface with the
shutdown and undo shutdown command.