line, there is a clocking problem. The Customer
Premises Equipment (CPE) will need to synchronize to the
clocking from the T1 provider. Complete the following steps to
correct this problem. Ensure that the clocking source is
provided by the telco. Review the output from the show
controllers t1 command to ensure that Clock Source is
Line. Note: If there are multiple T1s coming into an
access server, only one can be the primary source. The other
T1s derive the clock from the primary source. If there are
multiple T1s, ensure the T1 line designated as the primary
clock source is configured correctly. A second T1 line may also
be configured that provides clocking in case the primary source
goes down. To do this, use the clock source line
secondary command from controller configuration mode. Set
both the primary and secondary T1 clock source from controller
configuration mode. Router(config-controller)#clock source
line primary
Router(config-controller)#clock source
line secondary 1 Ensure that the T1s that were specified as
the primary and secondary are both active and stable. Ensure
the framing format configured on the port matches the framing
format of the line. Look for the statement Framing is {ESF|SF}
in the show controllers t1 output. To change the framing
format, use the framing {sf | esf} command in the
controller configuration mode. For example:
Router(config-controller)#framing esf Change the line
build-out using the cablelength long or cablelength
short command. Contact the Service Provider and consult the
Cisco web site documentation for details on build-out settings.
Basically, this command will change the signal levels used by
the T1 interface to allow for losses experienced due to the
distance between the WAN connection and the telco office. Line
codes are a method of encoding data such that there are regular
transitions between ones and zeros. This is necessary in a
digital environment because the ones and zeros are represented
by the presence or absence of a signal. Long sequences of ones
or zeros make it difficult for the controller to know just how
many ones or zeros are occurring. Figure shows an incoming
signal with regular transitions that is easy to interpret.
However, the long sequence of zero’s makes it more difficult.
In fact, the longer the sequence, the more accurate the clocks
must be and the more susceptible to errors the controller will
be. To avoid the difficulties associated with long sequences of
one’s and zero’s line coding algorithms are used to ensure that
regular one to zero and zero to one transitions occur. Ensure
that the line coding configured on the port matches the line
coding of the line. Look for the statement Line Code is
{B8ZS|AMI} in the show controllers t1 output. BZ8S stands for
Bipolar with 8 Zero Substitution. This means that continuous
sequences of zeros are replaced with a special BZ8S code that
includes transitions. BZ8S is used with Alternate Mark
Inversion (AMI) which encodes ones alternately as a positive
and negative voltages which is more compatible with
transformers and older cabling technologies used by telcos in
the local loop. To change the line coding, use the linecode
{ami | b8zs} command in controller configuration mode. For
example: Router(config-controller)#linecode b8zs Aside
for the necessity of correct encoding, the actual line voltage
levels also need to be within certain limits. The signal level
diminishes as the length of the local loop increases. This
variation can be compensated for with the cablelength
long or cablelength short commands. Path code
violations are frame synchronization errors when Super Frames
are in use, and cyclic redundancy check (CRC) errors for
Extended Super Frames. Path code violations and line code
violations are typically present simultaneously.
Content
4.4 Troubleshooting ISDN 4.4.2
Troubleshooting ISDN primary rate switch types In order to
communicate correctly a router must be told what type of ISDN
switching equipment is used by the telco. Depending on the
combination of telco equipment and the ISDN switch type
configuration, a variety of symptoms are possible. It is
usually quickest to simply make a careful check using the
show running-config command and verify that isdn
switch-type and pri-group timeslots are configured
correctly. To specify the central office switch type on the
ISDN interface, use the isdn switch-type global
configuration command. Options for this command include:
- primary-5ess
- primary-dms100
- primary-ni
Contact the Service
Provider for the correct values to use. To configure the
isdn switch-type and pri-group: Router#configure
terminal
Router(config)#isdn switch-type
primary-5ess
Router(config)#controller t1 0
Router(config-controller)#pri-group timeslots 1-24
Content 4.4 Troubleshooting ISDN
4.4.3 Troubleshooting ISDN BRI to telco
communications ISDN problems generally fall into one of the
following categories: - Communications problems between
the router and the telco’s ISDN switch
- Logical
configuration problems with dialers
- Problems with the
number being called
A divide and conquer approach is
the best way to troubleshoot ISDN. The show isdn
status command will quickly determine whether the router is
communicating correctly with telco’s ISDN switch. If there is a
problem in this regard, the show isdn status command
will often provide helpful diagnostic information. In
particular Layer 1 should be active, Layer 2 should have
MULTIPLE_FRAME_ESTABLISHED and SPIDs should be valid. If Layer
1 is deactivated then troubleshoot the following physical layer
problems: - Physical cabling
- Interface is
activated (no shutdown)
- ISDN Switch type
Like primary rate interfaces, basic rate interfaces must be
told what type of ISDN switching equipment is used by the
telco. The configured ISDN switch type can also be determined
with the show isdn status command. In Figure , no switch
type has been configured. A misconfigured switch type can
result in either Layer 1 or Layer 1/2 status “NOT Activated”.
In early versions of the Cisco IOS the switch type was
configured from global configuration, and a router could only
be used with one ISDN switch type service. Later versions of
the IOS allow the switch type to be specified at an interface
level, removing this constraint. If Layer 1 is functioning but
Layer 2 is inactive, then check the ISDN switch type. If SPID1
or SPID2 are not valid, carefully check the configured SPIDs
and verify these with the telco. It may be necessary to
shutdown the BRI and then reactivate it or reboot the router to
force SPID negotiation after the configuration is changed.
There is also a known bug in some versions of the IOS that
reports an invalid SPID even though the configured value is
correct. As long as the BRI is able to make calls, this
“cosmetic” bug should not be reason for concern. In certain
parts of the world, notably in Europe, telco ISDN switches may
deactivate Layer 1 or 2 when there are no active calls. Hence,
when there are no active calls, show isdn status will indicate
the Layer 1 and 2 are down. But when a call occurs, Layers 1
and 2 will be brought up. Make a test BRI call to verify
whether the BRI is functioning. If the call succeeds, then no
further ISDN troubleshooting is required. Layer 3 Status:
TWAIT timer active
0 Active Layer 3 Call(s)
Activated
dsl 0 CCBs = 0
The Free Channel Mask: 0x80000003
Total Allocated ISDN CCBs = 0 Reboot the ISDN router by cycling
the power. When the router comes up, the BRI interface will be
unable to establish a call for a random period of time.
Executing the show isdn status command will reveal that
the TWAIT timer is active. This is a feature that prevents a
telco’s ISDN switch from being overloaded after a major power