CSU/DSU Loopback TestingBy using loopback testing, you can isolate serial line problems. Four different loopback tests can be performed on a circuit to aid in troubleshooting it, including
The administrator of the local equipment can perform the first two tests, and the local service provider can perform the other two tests. In Figure 10.1, you see the device locations for the loopback tests to be performed. Figure 10.1. The four types of loopback tests used in troubleshooting serial links.
The Local Management Interface (LMI) is the signaling standard between the local router connecting to the CSU/DSU and the Central Office (CO) switch, and provides support for keepalive devices to verify data flow. The CO is the place where telephone companies terminate customer lines and locate switching equipment to interconnect those lines with other networks. Keepalives are packets that each device in the virtual circuit generates and sends to notify the other devices of connectivity. Just as their name suggests, keepalives "keep alive " a connection. When using loopback tests for troubleshooting, you should follow these steps:
If the local router's interface connecting to the CSU/DSU is working properly during a loopback test, then the local protocol and link should be functioning properly. When this is the case, however, it only confirms that the local equipment is functioning properly. This condition doesn't guarantee that the remote equipment is functioning properly, either. When putting the CSU/DSU into loopback mode, the signal is sent from the local router interface to the CSU/DSU and is immediately sent back by the CSU/DSU as though it were a remote signal coming back. When using the show interface command, the line protocol will show up in the output and it will appear that the connection is functioning normally. When configuring end-to-end connectivity to a remote site, both side's LMI (routers) status should say that the interface and protocol are up, and the LMI DLCI should state 1023, as shown in the following output using the show interfaces serial command: Sean2514# show interface serial 0 Serial0 is up, line protocol is up Hardware is HD64570 Internet address is 207.212.78.107/24 MTU 1500 bytes, BW 64 Kbit, DLY 20000 usec, rely 255/255, load 1/255 Encapsulation FRAME-RELAY IETF, loopback not set, keepalive set (10 sec) LMI enq sent 7107, LMI stat recvd 7164, LMI upd recvd 0, DTE LMI up LMI enq recvd 219, LMI stat sent 0, LMI upd sent 0 LMI DLCI 1023 LMI type is CISCO frame relay DTE Broadcast queue 0/64, broadcasts sent/dropped 1195/0, interface broadcasts 0 Last input 00:00:01, output 00:00:01, output hang never Last clearing of "show interface" counters 22:23:54 Input queue: 0/75/0 (size/max/drops); Total output drops: 0 Queueing strategy: weighted fair Output queue: 0/1000/64/0 (size/max total/threshold/drops) Conversations 0/1/256 (active/max active/max total) Reserved Conversations 0/0 (allocated/max allocated) 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 11501 packets input, 586321 bytes, 0 no buffer Received 888 broadcasts, 0 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort 9455 packets output, 184195 bytes, 0 underruns 0 output errors, 0 collisions, 51 interface resets 0 output buffer failures, 0 output buffers swapped out 124 carrier transitions DCD=up DSR=up DTR=up RTS=up CTS=up Sean2514# All the carrier operation switches (also known as the central office, or CO, switches) that participate in the permanent virtual circuit ( PVC ) or switched virtual circuit ( SVC ) provided by the local service provider must be working properly as well. An SVC is a virtual circuit that is established dynamically on demand to form a dedicated link and is then shut down when a transmission is complete. A PVC is a virtual circuit that, once connected, is active regardless of whether or not there is data to transmit. The local service provider can perform loopback tests on its equipment to confirm the functionality of the circuit. |