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: 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: 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: 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