1,Local provider problems
a,check the CSU/DSU for a CD signal. check for other signals, such as RX and TX clocking, to see if the circuit is transmitting and receiving information
b,if you do not get a CD signal or have other problems, contact the local service provider to troubleshoot and fix the problem
2,Improper or faulty cabling
a,make sure you are using the proper cable for the equipment being used
b,use a breakout box to check the control leads
c,swap faulty cables
3,Misconfigured interface
a,view the interface configuration using the show running-config command
b,make sure that the same encapsulation type is used at both ends of the circuit by using the show interface command
4,Keepalive problems
a,verify that keepalives are being sent. You can check this via the router configuration or by using the show interface command
b,if the configuration says that keepalives are being sent, you may want to enable debug serial interface for the interface
c,verify that the sequence numbers are incrementing
d,if the sequences don't increment, run loopback tests on the local and remote sites
e,if the sequences don't increment even when the CSU/DSU is in loopback, you have a hardware problem. Replace faulty hardware
5,Hardware failure
Replace the hardware
6,Interface is in loopback mode
a,check the interface configuration
b,if there is a loopback entry in the interface configuration, remove it with the no form of the command
c,if the interface ocnfiguration is clean, check the CSU/DSU to see if it is placed in loopback
d,if the CSU/DSU is in loopback, remove it from loopback mode
e,if the CSU/DSU is not in loopback mode, contact the local provider; it may have placed the circuit in loopback
7,Interface is administratively down
a,check the configuration. verify that the IP address is not a duplicate
b,enter the configuration mode and issue the no shutdown command within the interface
8,Line speed is larger than the interface capacity
a,reduce input queue size by using the hold-queue in command
b,increase output queues on exiting interfaces
9,Interface speed is larger than the line speed
a,reduce broadcast traffic
b,increase output queue
c,implement queuing algorithms, if necessary
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