The debug ppp negotiation command is useful for two reasons. First, it can enhance the
troubleshooting process on PPP links. Second, it provides a wonderful summary of how PPP
works, including LCP and the higher protocols. The higher protocols consist of IPCP (IP) and
CDPCP (CDP), among others.
The following output shows the messages that might appear when using the debug ppp
negotiation command:
Router#debug ppp negotiation
PPP protocol negotiation 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:
00:22:28: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up
00:22:28: BR0:1 PPP: Treating connection as a callout
00:22:28: BR0:1 PPP: Phase is ESTABLISHING, Active Open
00:22:28: BR0:1 LCP: O CONFREQ [Closed] id 3 len 10
00:22:28: BR0:1 LCP: MagicNumber 0x50239604
(0x050650239604)
00:22:28: BR0:1 LCP: I CONFREQ [REQsent] id 13 len 10
00:22:28: BR0:1 LCP: MagicNumber 0x5023961F
(0x05065023961F)
00:22:28: BR0:1 LCP: O CONFACK [REQsent] id 13 len 10
00:22:28: BR0:1 LCP: MagicNumber 0x5.023961F
(0x05065023961F)
00:22:28: BR0:1 LCP: I CONFACK [ACKsent] id 3 len 10
00:22:28: BR0:1 LCP: MagicNumber 0x50239604
(0x050650239604)
00:22:28: BR0:1 LCP: State is Open
00:22:28: BR0:1 PPP: Phase is UP
00:22:28: BR0:1 CDPCP: O CONFREQ [Closed] id 3 len 4
00:22:28: BR0:1 IPCP: O CONFREQ [Closed] id 3 len 10
00:22:28: BR0:1 IPCP: Address 10.1.1.2 (0x03060A010102)
00:22:28: BR0:1 CDPCP: I CONFREQ [REQsent] id 3 len 4
00:22:28: BR0:1 CDPCP: O CONFACK [REQsent] id 3 len 4
00:22:28: BR0:1 IPCP: I CONFREQ [REQsent] id 3 len 10
00:22:28: BR0:1 IPCP: Address 10.1.1.1 (0x03060A010101)
00:22:28: BR0:1 IPCP: O CONFACK [REQsent] id 3 len 10
00:22:28: BR0:1 IPCP: Address 10.1.1.1 (0x03060A010101)
00:22:28: BR0:1 CDPCP: I CONFACK [ACKsent] id 3 len 4
00:22:28: BR0:1 CDPCP: State is Open
00:22:28: BR0:1 IPCP: I CONFACK [ACKsent] id 3 len 10
00:22:28: BR0:1 IPCP: Address 10.1.1.2 (0x03060A010102)
00:22:28: BR0:1 IPCP: State is Open
00:22:28: BR0 IPCP: Install route to 10.1.1.1
Router#.!!!
Success rate is 60 percent (3/5), round-trip min/avg/max = 32/38/48 ms
00:22:29: %LINEPROTO-5-UPDOWN: Line protocol on Interface
BRI0:1, changed state to up
00:22:29: %LINK-3-UPDOWN: Interface BRI0:2, changed state to up
00:22:29: BR0:2 PPP: Treating connection as a callin
00:22:29: BR0:2 PPP: Phase is ESTABLISHING, Passive Open
00:22:29: BR0:2 LCP: State is Listen
00:22:30: BR0:2 LCP: I CONFREQ [Listen] id 3 len 10
00:22:30: BR0:2 LCP: MagicNumber 0x50239CC8
(0x050650239CC8)
00:22:30: BR0:2 LCP: O CONFREQ [Listen] id 3 len 10
00:22:30: BR0:2 LCP: MagicNumber 0x50239CDA
(0x050650239CDA)
00:22:30: BR0:2 LCP: O CONFACK [Listen] id 3 len 10
00:22:30: BR0:2 LCP: MagicNumber 0x50239CC8
(0x050650239CC8)
00:22:30: BR0:2 LCP: I CONFACK [ACKsent] id 3 len 10
00:22:30: BR0:2 LCP: MagicNumber 0x50239CDA
(0x050650239CDA) 00:22:30: BR0:2 LCP: State is Open
00:22:30: BR0:2 PPP: Phase is UP
00:22:30: BR0:2 CDPCP: O CONFREQ [Closed] id 3 len 4
00:22:30: BR0:2 IPCP: O CONFREQ [Closed] id 3 len 10
00:22:30: BR0:2 IPCP: Address 10.1.1.2 (0x03060A010102)
00:22:30: BR0:2 CDPCP: I CONFREQ [REQsent] id 3 len 4
00:22:30: BR0:2 CDPCP: O CONFACK [REQsent] id 3 len 4
00:22:30: BR0:2 IPCP: I CONFREQ [REQsent] id 3 len 10
00:22:30: BR0:2 IPCP: Address 10.1.1.1 (0x03060A010101)
00:22:30: BR0:2 IPCP: O CONFACK [REQsent] id 3 len 10
00:22:30: BR0:2 IPCP: Address 10.1.1.1 (0x03060A010101)
00:22:30: BR0:2 CDPCP: I CONFACK [ACKsent] id 3 len 4
00:22:30: BR0:2 CDPCP: State is Open
00:22:30: BR0:2 IPCP: I CONFACK [ACKsent] id 3 len 10
00:22:30: BR0:2 IPCP: Address 10.1.1.2 (0x03060A010102)
00:22:30: BR0:2 IPCP: State is Open
00:22:31: %LINEPROTO-5-UPDOWN: Line protocol on Interface BRI0:2, changed state
to up
00:22:32: BR0:1 LCP: O ECHOREQ [Open] id 12 len 12 magic
0x5020C645
00:22:32: BR0:1 LCP: echo_cnt 1, sent id 12, line up
00:22:32: BR0:1 PPP: I pkt type 0xC021, datagramsize 16
00:22:32: BR0:1 LCP: I ECHOREP [Open] id 12 len 12 magic
0x5020C654
00:22:32: BR0:1 LCP: Received id 12, sent id 12, line up
00:22:32: BR0:2 LCP: O ECHOREQ [Open] id 12 len 12 magic
0x5020CD1B
00:22:32: BR0:2 LCP: echo_cnt 1, sent id 12, line up
00:22:32: BR0:2 PPP: I pkt type 0xC021, datagramsize 16
00:22:32: BR0:2 LCP: I ECHOREP [Open] id 12 len 12 magic
0x5020CD0D
00:22:32: BR0:2 LCP: Received id 12, sent id 12, line up
00:22:33: BR0:1 PPP: I pkt type 0xC021, datagramsize 16
00:22:33: BR0:1 LCP: I ECHOREQ [Open] id 12 len 12 magic
0x5020C654
00:22:33: BR0:1 LCP: O ECHOREP [Open] id 12 len 12 magic
0x5020C64500:21:23: BR0:2 PPP: I pkt type 0xC021, datagramsize 16
00:22:33: BR0:2 LCP: I ECHOREQ [Open] id 12 len 12 magic
0x5020CD0D
00:22:33: BR0:2 LCP: O ECHOREP [Open] id 12 len 12 magic
0x5020CD1B
00:22:34: BR0:2 PPP: I pkt type 0x0207, datagramsize 15
00:22:35: BR0:2 PPP: I pkt type 0x0207, datagramsize 312
00:24:28: %ISDN-6-DISCONNECT: Interface BRI0:1 disconnected
from 18008358661 To p, call lasted 120 seconds
00:24:28: %LINK-3-UPDOWN: Interface BRI0:1, changed state to down
00:24:10: %ISDN-6-DISCONNECT: Interface BRI0:2 disconnected from 8358663, call
lasted 120 seconds
00:24:28: %LINK-3-UPDOWN: Interface BRI0:2, changed state to down
00:24:29: %LINEPROTO-5-UPDOWN: Line protocol on Interface
BRI0:1, changed state to down
00:24:29: %LINEPROTO-5-UPDOWN: Line protocol on Interface
BRI0:2, changed state to down
Notice that in this output, the first two ICMP packets (pings) failed due to the delay in bringing
up the ISDN BRI. Although faster than asynchronous connections, ISDN still introduces connection
delay, which can impact user applications. In addition, the output from the debug ppp
negotiation command shows the process by which a PPP session is activated.
This output does not use CHAP, compression, or multilink. Instead, as you can see, PPP
starts and then LCP is activated. After this occurs, the NCP negotiations begin, starting with
CDPCP and followed by IPCP. Cisco Discovery Protocol (CDP) is a proprietary advertisement
protocol that sends router and switch information between Cisco devices. It operates over any
physical media that supports Subnetwork Access Protocol (SNAP) (except ATM) and is independent
of IP. The IP Control Protocol (IPCP) was started to transport the ICMP pings that
were sent from the router.
Remember that PPP sessions must undergo a negotiation process and that the debug ppp
negotiation command will display upper level protocols such as IPCP, along with LCP and PPP.