When troubleshooting a Frame Relay connection, what is the first step when performing a loopback test?
A.
Set the encapsulation of the interface to HDLC.
B.
Place the CSU/DSU in local-loop mode.
C.
Enable local-loop mode on the DCE Frame Relay router.
D.
Verify that the encapsulation is set to Frame Relay.
Misleading question – this is only when layer 1 is down
“Serial0 is down, line protocol is down”
This output means you have a problem with the cable, channel service unit/data service unit (CSU/DSU), or the serial line. You need to troubleshoot the problem with a loopback test. To do a loopback test, follow the steps below:
Set the serial line encapsulation to HDLC and keepalive to 10 seconds. To do so, issue the commands encapsulation hdlc and keepalive 10 under the serial interface.
Place the CSU/DSU or modem in local loop mode. If the line protocol comes up when the CSU, DSU or modem is in local loopback mode (indicated by a “line protocol is up (looped)” message), it suggests that the problem is occurring beyond the local CSU/DSU. If the status line does not change states, there is possibly a problem in the router, connecting cable, CSU/DSU or modem. In most cases, the problem is with the CSU/DSU or modem.
Ping your own IP address with the CSU/DSU or modem looped. There should not be any misses. An extended ping of 0x0000 is helpful in resolving line problems since a T1 or E1 derives clock from data and requires a transition every 8 bits. B8ZS ensures that. A heavy zero data pattern helps to determine if the transitions are appropriately forced on the trunk. A heavy ones pattern is used to appropriately simulate a high zero load in case there is a pair of data inverters in the path. The alternating pattern (0x5555) represents a “typical” data pattern. If your pings fail or if you get cyclic redundancy check (CRC) errors, a bit error rate tester (BERT) with an appropriate analyzer from the telco is needed.
When you are finished testing, make sure you return the encapsulation to Frame Relay.
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