Cisco® CCNP Support : Troubleshooting Frame Relay, Isdn, And Other Protocols.. Cable Testers

16.0 Troubleshooting:

The first step in troubleshooting a device is to examine the physical indicators such as LEDs. Check if there is any visual hints. This may lead to a failed power supply or disconnected cable.
The next step in diagnosing a switch problem is to check Layer 1 connectivity, and then Layer 2 connectivity. The vlan and vtp information can be obtained and diagnosed at this point.

17.0 Frame Relay:

Important 'show' command used in troubleshooting Frame-Relay networks are given below:

1. Show interfaces serial - The information provided by this command include:
- LMI type (cisco/q933a/ansi
- LMI DLCI, Keepalives, Network address and subnet mask.
- Shows the interface and line protocol conditions (Normally interface up, line protocol up)

2. Show frame-relay lmi - Provides LMI statistics.

3. Show frame-relay map: This command shows the configured dlci and ip address mapping. 

4. Show frame-relay pvc: This command shows the LMI status of each configured DLCI. The packet stats for input pkts, output pkts, BECN, FECN, DE packet stats are also given.

18.0 ISDN:

ISDN can be mapped to bottom 3 layers of OSI reference model.
Layer 1 is the physical layer and the following protocols are defined at this level:

1. I.430: Defines the communication across S/T reference point.

2. I.431: Defines a full-duplex, point-to-point, serial, synchronous connection and applicable for ISDN PRI.

3. ANSI T1.601: Defines the communication across the U interface (North America only).
Layer 2 and layer 3 work for D channel only. 

Layer 2 of ISDN uses LAPD (Q.921), and Layer 3 of D channel uses Q.931 which is a subset of DSS1.
B channel uses IP/IPX with HDLC/PPP/FR/LAPB protocol.

Important ISDN commands:

debug isdn q921: This debug command shows the Layer 2 D-channel activity taking place between the TE (Terminal Equipment) and the local network termination (ISDN Switch).

Debug isdn q931: This debug command shows the Layer-3 D-channel information between the local router and the ISDN switch.

Debug ppp authentication: This debug command will display CHAP and PAP authentication messages that are being exchanged.

debug bri: shows basic isdn bri routing activity. It indicates whether the ISDN code is enabling and disabling the B-channels when attempting an out going call.

show isdn status - This command outputs ISDN switch type, whether the ISDN connection is working properly. It also displays the status of Layer 1, Layer 2, and layer 3.

Show interface bri 0 - This command shows whether BRI 0 is up and whether line protocol is up (spoofing). The command also displays encapsulation method used (PPP, HDLC, or X.25).

Note that D-channel communication takes place between the TE (Usually the router) and the local network termination point (ISDN Switch). ' debug isdn q921', and 'debug isdn q931' are limited to D-channel information only. Also, 'debug isdn q921' pertains to layer-2, where as 
'debug isdn q931' pertains to layer-3 (again D-channel only). These commands don't show B-channel debug information.

The BRI activation process is a 5 step process. These are:
1. The line is not active in both the directions between a TE and NT.
2. TE becomes active and sends 7E HDLC flag.
3. NT side sends frames with Activation bit (A-bit) set to 0 (indicating not activated)
4. TE synchronizes first and starts sending physical frames.
5. NT also synchronizes and starts sending frames with activation bit (A-bit) set to 1 (indicating activated)

19.0 Switching:

Netflow switching is most suitable for collecting data for accounting, billing, and charge back and other network resource utilization parameters.

20.0 Cable Testers:

Cable testers are the most basic testing tools. These are used for making sure that the physical medium is good. Cable test equipment include volt-ohm meters, multimeters, TDRs (Time Domain Reflectometers).

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