Troubleshooting PPP links

Negotiate LCP parameters

Lines 6-59 show the negotiation of Link Control Protocol (LCP) parameters.

6   Link link_0 : LCP FSM Event Administrative Open
7   Link link_0 : LCP      Causes State INITIAL -> STARTING
8   Link link_0 : LCP Starting
9   Link link_0 : LCP FSM Event Up
10  Link link_0 : LCP      Causes State STARTING -> REQSENT
11  Link link_0 : LCP Constructing Configure Request
12  Link link_0 : LCP Request MRU 1500
13  Link link_0 : LCP 1500 is the default MRU, don't request it
14  Link link_0 : LCP Request ACCM 0xFFFFFFFF
15  Link link_0 : LCP 0xFFFFFFFF is the default ACCM, don't request it
16  Link link_0 : LCP Request Magic number 0xF4855AC4
17  Link link_0 : LCP Request PFC
18  Link link_0 : LCP Request ACFC
19  Link link_0 : LCP SKIP MRRU - Not configured for MP
20  Link link_0 : LCP Send Configure Request
21  Link link_0 : LCP Received Configure Ack
22  Link link_0 : LCP FSM Event Receive Configure Ack
23  Link link_0 : LCP      Causes State REQSENT -> ACKRCVD
24  Link link_0 : LCP Received Configure Request
25  Link link_0 : LCP Peer requests ACCM 0x00000000 (Nak with 0xFFFFFFFF)
26  Link link_0 : LCP Peer Magic number 0xEE5B1C4F (Ack)
27  Link link_0 : LCP Configure Request is unacceptable (Nak)
28  Link link_0 : LCP FSM Event Receive Configure Request (Not acceptable)
29  Link link_0 : LCP      Causes State ACKRCVD -> ACKRCVD
30  Link link_0 : LCP Send Configure Nak/Rej
31  Link link_0 : LCP Received Configure Request
32  Link link_0 : LCP Peer requests ACCM 0xFFFFFFFF (Ack)
33  Link link_0 : LCP Peer Magic number 0x8722C26A (Ack)
34  Link link_0 : LCP Peer requests PFC (Ack)
35  Link link_0 : LCP Peer requests ACFC (Ack)
36  Link link_0 : LCP Setting default MRU
37  Link link_0 : LCP Default AUTH (none)
38  Link link_0 : LCP Default MRRU (none)
39  Link link_0 : LCP Default ED (none)
40  Link link_0 : LCP Configure Request is acceptable (Ack)
41  Link link_0 : LCP FSM Event Receive Configure Request (Acceptable)
42  Link link_0 : LCP      Causes State ACKRCVD -> OPENED
43  Link link_0 : LCP Send Configure Ack
44  Link link_0 : LCP Up
45  Link link_0 : LCP Local options :
46  Link link_0 : LCP    Mru 1500
47  Link link_0 : LCP    Accm 0xffffffff
48  Link link_0 : LCP    ACFC Enabled
49  Link link_0 : LCP    PFC Enabled
50  Link link_0 : LCP    Magic Number 0xf4855ac4
51  Link link_0 : LCP    MultiLink Disabled
52  Link link_0 : LCP Peer options :
53  Link link_0 : LCP    Mru 1500
54  Link link_0 : LCP    Accm 0xffffffff
55  Link link_0 : LCP    ACFC Enabled
56  Link link_0 : LCP    PFC Enabled
57  Link link_0 : LCP    Magic Number 0x8722c26a
58  Link link_0 : LCP    MultiLink Disabled
59  Link link_0 : LCP Sending LCP Identification

On lines 24-30, the local host indicates to the peer that it wants to use an asynchronous control character map (ACCM) of 0xFFFFFFFF rather than the peer's suggested ACCM of 0x00000000. As hardware rather than software flow control is being used with the modems at both ends of the link, it would be possible to set the local LCP configuration to request an all-zero map. This would improve the performance of data transfer over the link.

On line 59, PPP sends an LCP identification packet to the remote host. A reply is not received until the IPCP phase of the negotiation on line 84 (see ``Negotiate IPCP and other parameters'').

Lines 45-58 summarize the negotiated LCP parameters. If the remote side requires that the local side authenticate itself using PAP or CHAP, you would also see one of the following lines in the peer options:

   LCP Require PAP Authentication
   LCP Require CHAP Authentication

If the local side requires that the remote side authenticate itself, one of these lines would appear in the local options.

Peer refuses to authenticate itself

If you see one on the following messages, the remote side has stated that it is not configured to authenticate itself using PAP or CHAP:

   LCP Peer Nak'ed request for PAP
   LCP Peer Nak'ed request for CHAP

The connection would then be closed, and one of the following messages would also be output:

   LCP We require PAP. Peer wouldn't authenticate using PAP.
   LCP We require CHAP. Peer wouldn't authenticate using CHAP.

See ``Perform PAP or CHAP authentication'' for more information about troubleshooting PAP and CHAP authentication once the local and remote sides have agreed on the method to be used.

Remote side does not start PPP session

If you see output such as the following when LCP sends its first configuration request to the remote side, it is likely that the remote side has not recognized that it should start a PPP session:

20  Link link_0 : LCP Send Configure Request
21  Link link_0 : LCP Restart Timer Expired (count 9)
22  Link link_0 : LCP FSM Event Timeout (Count non-zero)
23  Link link_0 : LCP      Causes State REQSENT -> REQSENT
24  Link link_0 : LCP Re-Send Configure Request
.
.
.
56  Link link_0 : LCP Re-Send Configure Request
57  Link link_0 : LCP Restart Timer Expired (count 0)
58  Link link_0 : LCP FSM Event Timeout (Count Zero)
59  Link link_0 : LCP      Causes State REQSENT -> STOPPED
60  Link link_0 : LCP Finished

If an outgoing link fails in this manner, it may be because it needs to issue an explicit command in its login chat script to start a PPP shell.