Troubleshooting the Network Layer


ISDN uses the q.931 signaling that is defined in the ITU-T I.451 to connect at Layer 3 of the OSI model. This signaling is responsible for a call's setup and disconnection using the D channel of an ISDN connection. Quite a few message commands exist, which can be viewed by using the debug isdn q931 command. These messages include the following:

  • Call setup

  • Connect

  • Release

  • Cancel

  • Status

  • Disconnect

  • User information

The q.931 signaling operates on the D channel, and the debug isdn q931 command includes information on the reference flag, message types, and information elements.

Call Reference Flag Definitions and Message Types

The two reference flag values are 0 and 1. The reference flag value 0 indicates that this is a call from another oriSeantor. Reference flag value 1 indicates that this is a call to an originator.

The following are the three different message type values associated with the q.931 protocol:

  • 0x05 ” Setup message type

  • 0x45 ” Disconnect message type

  • 0x7d ” Status message type

Information Elements

When troubleshooting q.931 signaling at Layer 3, information elements are included in the output that can help identify problems. The following are some information elements and what they mean:

  • 0x04 ” Bearer capability

  • 0x2c ” Keypad facility

  • 0x6c ” Calling party number

  • 0x70 ” Called party number

  • 0x3a ” SPID

Call Setup Steps for q.931 Signaling

ISDN establishes a Layer 3 connection between the local ISDN router and the local service provider's switch over the D channel. The local service provider's switch establishes another connection to the remote switch, which in turn is responsible for setting up a call to the remote ISDN router.

Before you look at the output from debug isdn q931 command, review the following to get an understanding of some of the pieces of the output and what they mean:

  • ALERT ” An alert from the remote TE to the local TE with a ring-back signal.

  • SETUP ” The SETUP process sends information for the connection between the local TE and the remote TE.

  • Call Proceeding (CALL_PROC) ” A call-proceeding signal between the ET and the TE.

  • CONNECT ” The remote TE acknowledges a call and stops the local ring-back process.

  • Connect Acknowledgment (CONNECT_ACK) ” An acknowledgment of the remote exchange termination point to the remote Terminal Equipment indicating the setup is complete.

The following is the output of the debug isdn q931 command:

 Seans804#  debug isdn q931  ISDN Q931 packets debugging is on 12:13:324523: ISDN BR0: RX <- STATUS_ENQ pd = 8 callref = 0x82 12:13:47: ISDN BR0: TX -> STATUS pd = 8  callref = 0x02 12:13:47: Cause i = 0x809E - Response to STATUS  ENQUIRY or number unassigned 12:13:47: Call State i = 0x0A 12:13:3246744: ISDN BR0: RX <- STATUS_ENQ pd = 8 callref = 0x06 12:13:47: ISDN BR0: TX ->  STATUS pd = 8  callref = 0x86 12:13:47: Cause i = 0x809E - Response to STATUS  ENQUIRY or number unassigned 12:13:47: Call State i = 0x0A 12:13:3246951: ISDN BR0: RX <- STATUS_ENQ pd = 8 callref = 0x82 12:13:47: ISDN BR0: TX ->  STATUS pd = 8  callref = 0x02 12:13:47: Cause i = 0x809E - Response to STATUS  ENQUIRY or number unassigned 12:13:49: Call State i = 0x0A 12:13:3247341: ISDN BR0: RX <- STATUS_ENQ pd = 8 callref = 0x06 12:13:49: ISDN BR0: TX ->  STATUS pd = 8  callref = 0x86 12:13:49: Cause i = 0x809E - Response to STATUS     ENQUIRY or number unassigned 12:13:49: Call State i = 0x0A 12:13:3248465: ISDN BR0: RX <- DISCONNECT pd = 8 callref = 0x82 12:13:3248465: Cause i = 0x8290 - Normal call    clearing 12:13:3248465: Signal i = 0x3F - Tones off 12:13:49: %ISDN-6-DISCONNECT:  Interface BRI0:1 disconnected from 18005551001  To p, call lasted 120 seconds 12:13:49: %LINK-3-UPDOWN: Interface BRI0:1, changed state to down 12:13:49: ISDN BR0: TX ->  RELEASE pd = 8  callref = 0x02 12:13:3248465: ISDN BR0: RX <-  RELEASE_COMP pd = 8 callref = 0x82 12:13:3248465: %ISDN-6-DISCONNECT:  Interface BRI0:2 disconnected from 8358 663, call lasted 120 seconds 12:13:49: ISDN BR0: TX ->  DISCONNECT pd = 8  callref = 0x86 12:13:49: Cause i = 0x8090 - Normal call clearing 12:13:3248465: ISDN BR0: RX <- RELEASE pd = 8 callref = 0x06 12:13:49: %LINK-3-UPDOWN: Interface BRI0:2, changed state to down 12:13:49: ISDN BR0: TX ->   RELEASE_COMP pd = 8  callref =   0x86 12:13:50: %LINEPROTO-5-UPDOWN: Line protocol on Interface     BRI0:1, changed state to down 12:13:50: %LINEPROTO-5-UPDOWN: Line protocol on Interface     BRI0:2, changed state to down 

The above debug output shows the complete q.931 process, from activating the interface, sending the data, and then disconnecting.

Correct Switch Types

ISDN uses a connection between the ISDN router and the phone company's central office switch. Both sides of the connection must be configured with the same switch type. If the switch types are different, it is like putting two people in the same room who speak different languages. The isdn switch-type command can be used in both the global and interface configuration modes. The following output from the help command lists the different switch types that are supported on the Cisco 804 ISDN router:

 Seans804(config)#  isdn switch-type ?  basic-1tr6    1TR6 switch type for Germany   basic-5ess    AT&T 5ESS switch type for the U.S.   basic-dms100  Northern DMS-100 switch type   basic-net3    NET3 switch type for UK and Europe   basic-ni      National ISDN switch type   basic-ts013   TS013 switch type for Australia   ntt           NTT switch type for Japan   vn3           VN3 and VN4 switch types for France 

You can use the isdn autodetect command to automatically detect the switch type if you do not know the correct switching type.

Many routers don't support the auto-detect feature, but it is supported on the Cisco 804 router, most ISDN routers, and most routers with BRI modules so long as the IOS version you install supports it. The switch type is only specific to the local loop switch, and not on the remote side of the connection, which can use a completely different switch type.



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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