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.
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)
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).