The debug ppp packet command reports real-time PPP packet flow, including the type of
packet and the specific B channel used in the case of ISDN. Although this command generates
a significant amount of output and could slow the access server, it is quite useful for locating
errors that involve upper layer protocols.
As with other debug protocol packet commands, the debug ppp packet command
records each packet that moves through the router using PPP. As such, the administrator can
monitor traffic flows as if they had a protocol analyzer attached to the interface. This might be
useful for troubleshooting Application layer problems, but a formal protocol analyzer is highly
recommended. This output includes both CDP packets (shown with the CDPCP entries) and IP
packets (showing proper configuration of IP on the link):
Router#debug ppp packet
PPP packet display debugging is on
Router#ping 10.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds:
00:24:49: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up.
00:24:50: BR0:1 LCP: O CONFREQ [Closed] id 4 len 10
00:24:50: BR0:1 LCP: MagicNumber 0x5025BF23
(0x05065025BF23)
00:24:50: BR0:1 PPP: I pkt type 0xC021, datagramsize 14
00:24:50: BR0:1 PPP: I pkt type 0xC021, datagramsize 14
00:24:50: BR0:1 LCP: I CONFREQ [REQsent] id 14 len 10
00:24:50: BR0:1 LCP: MagicNumber 0x5025BF46
(0x05065025BF46)
00:24:50: BR0:1 LCP: O CONFACK [REQsent] id 14 len 10
00:24:50: BR0:1 LCP: MagicNumber 0x5025BF46
(0x05065025BF46)
00:24:50: BR0:1 LCP: I CONFACK [ACKsent] id 4 len 10
00:24:50: BR0:1 LCP: MagicNumber 0x5025BF23
(0x05065025BF23)
00:24:50: BR0:1 PPP: I pkt type 0x8207, datagramsize 8
00:24:50: BR0:1 PPP: I pkt type 0x8021, datagramsize 14
00:24:50: BR0:1 CDPCP: O CONFREQ [Closed] id 4 len 4
00:24:50: BR0:1 PPP: I pkt type 0x8207, datagramsize 8
00:24:50: BR0:1 IPCP: O CONFREQ [Closed] id 4 len 10
00:24:50: BR0:1 IPCP: Address 10.1.1.2 (0x03060A010102)
00:24:50: BR0:1 CDPCP: I CONFREQ [REQsent] id 4 len 4
00:24:50: BR0:1 CDPCP: O CONFACK [REQ.!!!
Success rate is 60 percent (3/5), round-trip min/avg/max = 36/41/52 ms
Router#sent] id 4 len 4
00:24:50: BR0:1 PPP: I pkt type 0x8021, datagramsize 14
00:24:50: BR0:1 IPCP: I CONFREQ [REQsent] id 4 len 10
00:24:50: BR0:1 IPCP: Address 10.1.1.1 (0x03060A010101)
00:24:50: BR0:1 IPCP: O CONFACK [REQsent] id 4 len 10
00:24:50: BR0:1 IPCP: Address 10.1.1.1 (0x03060A010101)
00:24:50: BR0:1 CDPCP: I CONFACK [ACKsent] id 4 len 4
00:24:50: BR0:1 IPCP: I CONFACK [ACKsent] id 4 len 10
00:24:50: BR0:1 IPCP: Address 10.1.1.2 (0x03060A010102)
00:24:51: BR0:1 PPP: O pkt type 0x0021, datagramsize 104
00:24:51: %LINEPROTO-5-UPDOWN: Line protocol on Interface
BRI0:1, changed state to up
00:24:51: BR0:1 PPP: O pkt type 0x0207, datagramsize 323
00:24:51: %LINK-3-UPDOWN: Interface BRI0:2, changed state to up
00:24:51: BR0:2 PPP: I pkt type 0xC021, datagramsize 14
00:24:51: BR0:2 LCP: I CONFREQ [Listen] id 4 len 10
00:24:51: BR0:2 LCP: MagicNumber 0x5025C5EF
(0x05065025C5EF)
00:24:51: BR0:2 LCP: O CONFREQ [Listen] id 4 len 10 00:24:51:
➥BR0:2 LCP: MagicNumber 0x5025C605
(0x05065025C605)
00:24:51: BR0:2 LCP: O CONFACK [Listen] id 4 len 10
00:24:51: BR0:2 LCP: MagicNumber 0x5025C5EF
(0x05065025C5EF)
00:24:51: BR0:2 PPP: I pkt type 0xC021, datagramsize 14
00:24:51: BR0:2 LCP: I CONFACK [ACKsent] id 4 len 10
00:24:51: BR0:2 LCP: MagicNumber 0x5025C605
(0x05065025C605)
00:24:51: BR0:2 PPP: I pkt type 0x8207, datagramsize 8
00:24:51: BR0:2 PPP: I pkt type 0x8021, datagramsize 14
00:24:51: BR0:2 CDPCP: O CONFREQ [Closed] id 4 len 4
00:24:51: BR0:2 IPCP: O CONFREQ [Closed] id 4 len 10
00:24:51: BR0:2 IPCP: Address 10.1.1.2 (0x03060A010102)
00:24:51: BR0:2 CDPCP: I CONFREQ [REQsent] id 4 len 4
00:24:51: BR0:2 CDPCP: O CONFACK [REQsent] id 4 len 4
00:24:51: BR0:2 PPP: I pkt type 0x8207, datagramsize 8
00:24:51: BR0:2 IPCP: I CONFREQ [REQsent] id 4 len 10
00:24:51: BR0:2 IPCP: Address 10.1.1.1 (0x03060A010101)
00:24:51: BR0:2 PPP: I pkt type 0x8021, datagramsize 14
00:24:51: BR0:2 IPCP: O CONFACK [REQsent] id 4 len 10
00:24:51: BR0:2 IPCP: Address 10.1.1.1 (0x03060A010101)
00:24:51: BR0:2 CDPCP: I CONFACK [ACKsent] id 4 len 4
00:24:51: BR0:2 IPCP: I CONFACK [ACKsent] id 4 len 10
00:24:51: BR0:2 IPCP: Address 10.1.1.2 (0x03060A010102)
00:24:52: BR0:1 LCP: O ECHOREQ [Open] id 1 len 12 magic
0x5025BF23
00:24:52: BR0:1 LCP: echo_cnt 1, sent id 1, line up
00:24:52: BR0:1 PPP: I pkt type 0xC021, datagramsize 16
00:24:52: BR0:1 LCP: I ECHOREP [Open] id 1 len 12 magic
0x5025BF46
00:24:52: BR0:1 LCP: Received id 1, sent id 1, line up
00:24:52: BR0:2 LCP: O ECHOREQ [Open] id 1 len 12 magic
0x5025C605
00:24:52: BR0:2 LCP: echo_cnt 1, sent id 1, line up
00:24:52: BR0:2 PPP: I pkt type 0xC021, datagramsize 16 00:24:52:
➥BR0:2 LCP: I ECHOREP [Open] id 1 len 12 magic
0x5025C5EF
00:24:52: BR0:2 LCP: Received id 1, sent id 1, line up
00:24:52: %LINEPROTO-5-UPDOWN: Line protocol on Interface
BRI0:2, changed state to up
00:24:52: BR0:1 PPP: O pkt type 0x0207, datagramsize 323
00:24:52: BR0:2 PPP: I pkt type 0x0207, datagramsize 312
00:24:53: BR0:1 PPP: O pkt type 0x0021, datagramsize 104
00:24:53: BR0:2 PPP: I pkt type 0x0021, datagramsize 104
00:24:53: BR0:1 PPP: O pkt type 0x0021, datagramsize 104
00:24:53: BR0:2 PPP: I pkt type 0x0021, datagramsize 104
00:24:53: BR0:1 PPP: O pkt type 0x0021, datagramsize 104
00:24:53: BR0:2 PPP: I pkt type 0x0021, datagramsize 104
00:24:53: BR0:1 PPP: I pkt type 0xC021, datagramsize 16
00:24:53: BR0:1 LCP: I ECHOREQ [Open] id 1 len 12 magic
0x5025BF46
00:24:53: BR0:1 LCP: O ECHOREP [Open] id 1 len 12 magic
0x5025BF23
00:24:53: BR0:2 PPP: I pkt type 0xC021, datagramsize 16
00:24:53: BR0:2 LCP: I ECHOREQ [Open] id 1 len 12 magic
0x5025C5EF
00:24:53: BR0:2 LCP: O ECHOREP [Open] id 1 len 12 magic
0x5025C605
Router#
00:25:02: BR0:1 LCP: O ECHOREQ [Open] id 2 len 12 magic
0x5025BF23
00:25:02: BR0:1 LCP: echo_cnt 1, sent id 2, line up
00:25:02: BR0:1 PPP: I pkt type 0xC021, datagramsize 16
00:25:02: BR0:1 LCP: I ECHOREP [Open] id 2 len 12 magic
0x5025BF46
00:25:02: BR0:1 LCP: Received id 2, sent id 2, line up
00:25:02: BR0:2 LCP: O ECHOREQ [Open] id 2 len 12 magic
0x5025C605
00:25:02: BR0:2 LCP: echo_cnt 1, sent id 2, line up
00:25:02: BR0:2 PPP: I pkt type 0xC021, datagramsize 16
00:25:02: BR0:2 LCP: I ECHOREP [Open] id 2 len 12 magic
0x5025C5EF
00:25:02: BR0:2 LCP: Received id 2, sent id 2, line up
00:25:03: BR0:1 PPP: I pkt type 0xC021, datagramsize 16
00:25:03: BR0:1 LCP: I ECHOREQ [Open] id 2 len 12 magic
0x5025BF46
00:25:03: BR0:1 LCP: O ECHOREP [Open] id 2 len 12 magic
0x5025BF23
00:25:03: BR0:2 PPP: I pkt type 0xC021, datagramsize 16
00:25:03: BR0:2 LCP: I ECHOREQ [Open] id 2 len 12 magic
0x5025C5EF
00:25:03: BR0:2 LCP: O ECHOREP [Open] id 2 len 12 magic
0x5025C605
The debug ppp packet command is most helpful in locating upper layer protocol errors.
It filters out non-PPP output, resulting in a cleaner debug output than a regular debug ip
packet command. Note that the magic numbers referred to in the previous output are used to
thwart playback attacks by maintaining a form of state for the session.