Tracing PPP Frames

When the ppptrace command is executed without arguments it displays a trace of all the active PPP links. You can restrict the trace by specifying an IP interface, or the link name you assigned to the link using the ppp_link_name parameter in the file ppp.conf.

To display a trace of the link associated with the IP interface ipdptp0, type:

prompt# ppptrace ipdptp0

To display a trace of the link associated with the link name ppplink0, type:

prompt# ppptrace ppplink0

The basic output from a typical ppptrace has the following components:

timestamp: link_id type: description [id: xx] [length: yyyy]

timestamp

The time at which the trace was recorded.

link_id

Identifies the link from which the trace was recorded. This may be an IP interface name (ipdptpn or ipdn) or a link name assigned using the ppp_link_name parameter in the file ppp.conf.

type

Identifies the source of the frame, which may be either sent by the local host (outgoing), or received from the remote host (incoming).

description

Describes the PPP frame type. See Appendix A, PPP Link Operation for more information.

id: xx

The contents of the id field of the PPP frame, which carries an identifier that is used to match associated requests and replies. See Appendix A, PPP Link Operation for more information.

length: yyyy

The contents of the length field of the PPP frame, which carries the total length of the information field. The length must be less than the maximum receive unit (MRU). See Appendix A, PPP Link Operation for more information.

For example, the following trace shows an LCP Configure-request packet sent by the local host. The basic trace is followed by information that describes the frame in more detail, including the magic number, which identifies the initial source of the frame and is used to detect looped back conditions. See Appendix A, PPP Link Operation for more information.

18:10:32: ipdptp0 Send: LCP Packet Config Request  [id: 15]
[length: 0012]
          Maximun Receive unit: 1500
          Magic Number: 4e0d20fb
          Protocol Field Compression
          Address and Control Field Compression