Troubleshooting the Data Link Layer: q.921 and PPP


Layer 2 of the OSI Reference model has two protocols that need to be working properly, if they are configured: q.921 and PPP. There is also a service access point identifier associated with q.921.

The q.921 Protocol

The q.921 protocol defines the signaling method used by ISDN at Layer 2 of the OSI Reference model. The q.921 protocol uses the D channel by using the Link Access Procedure on D channel (LAPD) protocol, which is used by X.25. If the q.921 connection between the CO switch and the local ISDN router does not occur and complete, there can be no network (Layer 3) connectivity.

If you use the show isdn status command and find that a Layer 2 problem exists, the best way to troubleshoot q.921 protocol problems is to use the debug isdn q921 command. Usually, a problem with q.921 relates to the terminal end point identifier (TEI). The TEI value uniquely identifies every terminal in the network. A value of 127 represents the broadcast address to all the terminals. TEIs 64 through 126 are reserved for use after the completion of a Layer 2 ISDN connection. You cannot assign this value, because it is a dynamic assignment.

The following output is from the show isdn status command on an incorrectly configured interface:

 Seans804#  show isdn status  Global ISDN Switchtype = basic-ni ISDN BRI0 interface dsl 0, interface ISDN Switchtype = basic-ni     Layer 1 Status: ACTIVE     Layer 2 Status: TEI = 79, Ces = 1, SAPI = 0, State = MULTIPLE_FRAME_   ESTABLISHED     Spid Status: TEI 79, ces = 1, state = 8(established)  spid1 configured, no LDN, spid1 NOT sent, spid1 NOT valid   TEI Not Assigned, ces = 2, state = 1(terminal down)   spid2 configured, no LDN, spid2 NOT sent, spid2 NOT valid  Layer 3 Status: 0 Active Layer 3 Call(s)     Activated dsl 0 CCBs = 1 CCB:callid=0x0, sapi=0x0, ces=0x1, B-chan=0 calltype = INTERNAL Total Allocated ISDN CCBs = 1 

The output above shows that the ISDN connection was established, but the SPIDs configured are invalid, as shown in the bolded lines.

The following output is correctly configured and shows that all the layers are active and configured correctly:

 Seans804#  show isdn status  Global ISDN Switchtype = basic-ni ISDN BRI0 interface         dsl 0, interface ISDN Switchtype = basic-ni     Layer 1 Status:         ACTIVE     Layer 2 Status:         TEI = 105, Ces = 1, SAPI = 0, State = MULTIPLE_FRAME_ESTABLISHED     Spid Status:         TEI 105, ces = 1, state = 5(init)             spid1 configured, no LDN, spid1 sent, spid1 valid             Endpoint ID Info: epsf = 0, usid = 3, tid = 1     Layer 3 Status:         2 Active Layer 3 Call(s)     Activated dsl 0 CCBs = 0 

Watch for Layer 3 Signaling and SABME Messages

You can use the debug isdn q921 command to identify the process that is running when a failure occurs. One thing to watch for in the output of the debug isdn q921 command is the SAPI, or service access point identifier, field. If this field lists a SAPI of 0, then Layer 3 signaling is present. This signaling is provided by q.931, which is covered later in this chapter, under the section "Troubleshooting the Network Layer." The value of 63 indicates a management SAPI for the assignment of the TEI values. A value of 64 indicates that q921 is using call control.

Another field to look at is the set asynchronous balanced mode extended (SABME) messages. SABME messages are exchanged with TEI message type 7. Should the SABME fail, a disconnect response message is sent and the link establishment is terminated . In this situation, you must determine the reason for the SABME failure.

When the SABME succeeds, an acknowledgment is sent that a Layer 2 connection is established. When this occurs, you should see the TE begin to send INFO-type frames .

PPP (Point-to-Point Protocol)

To troubleshoot the PPP protocol and isolate CHAP problems, you should first check both sides of the ISDN link to verify that the usernames and passwords are configured correctly. Knowing the steps that PPP and CHAP use in negotiating a connection can also aid in determining the problem. The following are the steps taken by PPP and CHAP to establish a link. These steps are sometimes referred to as targets.

  1. The TE sends a CONFREQ, which is a configuration request specifying certain router options.

  2. If the request is accepted, a CONFACK is sent back to the TE, which is an acknowledgment. If the request is denied , a CONFREJ is sent to the requesting TE.

  3. If CHAP is used for authentication, it continues to steps 4 through 6, which are the three-way handshake process.

  4. A challenge message is sent to the remote TE.

  5. The remote TE sends a response message.

  6. If the response values match, authentication is given.

The following is an example of using the debug ppp negotiation command, which is extremely helpful in researching PPP problems and resolving them:

 Seans804#  debug ppp negotiation  PPP protocol negotiation debugging is on Seans804#  ping 172.16.1.1  Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.16.1.1, timeout is 2    seconds: 02:26:33: %LINK-3-UPDOWN: Interface BRI0:1, changed state to    up 02:26:33: BR0:1 PPP: Treating connection as a callout 02:26:33: BR0:1 PPP: Phase is ESTABLISHING, Active Open 02:26:33: BR0:1 LCP: O CONFREQ [Closed] id 3 len 10 02:26:33: BR0:1 LCP:    MagicNumber 0x50239604    (0x050650239604) 02:26:33: BR0:1 LCP: I CONFREQ [REQsent] id 13 len 10 02:26:33: BR0:1 LCP:    MagicNumber 0x5023961F    (0x05065023961F) 02:26:33: BR0:1 LCP: O CONFACK [REQsent] id 13 len 10 02:26:33: BR0:1 LCP:    MagicNumber 0x5.023961F    (0x05065023961F) 02:26:33: BR0:1 LCP: I CONFACK [ACKsent] id 3 len 10 02:26:33: BR0:1 LCP:    MagicNumber 0x50239604    (0x050650239604) 02:26:33: BR0:1 LCP: State is Open 02:26:33: BR0:1 PPP: Phase is UP 02:26:33: BR0:1 CDPCP: O CONFREQ [Closed] id 3 len 4 02:26:33: BR0:1 IPCP: O CONFREQ [Closed] id 3 len 10 02:26:33: BR0:1 IPCP:    Address 10.1.1.2 (0x03060A010102) 02:26:33: BR0:1 CDPCP: I CONFREQ [REQsent] id 3 len 4 02:26:33: BR0:1 CDPCP: O CONFACK [REQsent] id 3 len 4 02:26:33: BR0:1 IPCP: I CONFREQ [REQsent] id 3 len 10 02:26:33: BR0:1 IPCP:    Address 172.16.1.1 (0x03060A010101) 02:26:33: BR0:1 IPCP: O CONFACK [REQsent] id 3 len 10 02:26:33: BR0:1 IPCP:    Address 10.1.1.1 (0x03060A010101) 02:26:33: BR0:1 CDPCP: I CONFACK [ACKsent] id 3 len 4 02:26:33: BR0:1 CDPCP: State is Open 02:26:33: BR0:1 IPCP: I CONFACK [ACKsent] id 3 len 10 02:26:33: BR0:1 IPCP:    Address 172.16.1.2 (0x03060A010102) 02:26:33: BR0:1 IPCP: State is Open 02:26:33: BR0 IPCP: Install route to 172.16.1.1 !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 32/38/48 ms 

The above output shows the entire PPP negotiation process for authenticating an ISDN connection. You will notice that the interesting traffic that activated the ISDN connection was an ICMP ping.

The debug ppp packet command reports the real-time PPP packet flow, including the type of packet and the B channel that is being used. As with other debug packet commands, the debug ppp packet command records each packet and can be used to monitor traffic flows. The following is the output generated from a successful ping:

 Sean804#  debug ppp packet  PPP packet display debugging is on Seans804#ping 172.16.1.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.16.1.1, timeout is 2 seconds: 01:11:19: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up. 01:11:19: BR0:1 LCP: O CONFREQ [Closed] id 4 len 10 01:11:19: BR0:1 LCP:    MagicNumber 0x5025BF23 (0x05065025BF23) 01:11:19: BR0:1 PPP: I pkt type 0xC021, datagramsize 14 01:11:19: BR0:1 PPP: I pkt type 0xC021, datagramsize 14 01:11:19: BR0:1 LCP: I CONFREQ [REQsent] id 14 len 10 01:11:19: BR0:1 LCP:    MagicNumber 0x5025BF46    (0x05065025BF46) 01:11:19: BR0:1 LCP: O CONFACK [REQsent] id 14 len 10 01:11:19: BR0:1 LCP:    MagicNumber 0x5025BF46    (0x05065025BF46) 01:11:19: BR0:1 LCP: I CONFACK [ACKsent] id 4 len 10 01:11:19: BR0:1 LCP:    MagicNumber 0x5025BF23    (0x05065025BF23) 01:11:20: BR0:1 PPP: I pkt type 0x8207, datagramsize 8 01:11:20: BR0:1 PPP: I pkt type 0x8021, datagramsize 14 01:11:20: BR0:1 CDPCP: O CONFREQ [Closed] id 4 len 4 01:11:20: BR0:1 PPP: I pkt type 0x8207, datagramsize 8 01:11:20: BR0:1 IPCP: O CONFREQ [Closed] id 4 len 10 01:11:20: BR0:1 IPCP:    Address 10.1.1.2 (0x03060A010102) 01:11:20: BR0:1 CDPCP: I CONFREQ [REQsent] id 4 len 4 


CCNP CIT Exam Cram 2 (642-831)
CCNP CIT Exam Cram 2 (Exam Cram 642-831)
ISBN: 0789730219
EAN: 2147483647
Year: 2003
Pages: 213
Authors: Sean Odom

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