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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