Loopback Connector Information

Serial Loopback Plugs

Figure 1:  Serial Loopback Plugs

T1 & E1 RJ48 loopback plug (99-7912-042-1) - (See Figure 2.)
1 loops back on to 4
2 loops back onto 5

T1/56K loopback plug (RJ48-RJ48 8wire, 99-7912-043-1)
1 loops back on to 7
2 loops back onto 8

Figure 2:  Frequently Used Loopback Connectors

Ethernet Loopback Plug

Ethernet RJ45 loopback plug
1 loops back on to 3
2 loops back onto 6

Using the Serial loopback plugs with Cisco Routers

Step 1   Issue a show interface command on the suspect Cisco serial interface. If the interface is "down/down", go to Step 2.

Step 2   Put loopback plug on Cisco Serial port. The show interface output should now show "up/down".

Step 3   In enable mode at the router prompt firstly save the config by issuing the write mem command. If there is no IP address for the interface, then provide one as shown below and enter the following (where 0 is the interface number in question):

conf t
int s 0
ip address 10.10.1.1 255.255.255.0
encapsulation hdlc
clock rate 64000

Step 4   Check status with show interface s 0.

Step 5   You should now see that the interface shows as "up/up (looped)", which indicates that the Cisco serial hardware is good.

Step 6  Ping this router's own interface IP address repeatedly to confirm the serial hardware.

Using the Ethernet loopback plug with Cisco Routers

Step 1   Do a "show interface" command on the suspect Cisco ethernet interface. If the interface is "up/down", go to Step 2.

Step 2   Put a loopback plug on the suspect Cisco ethernet port. The show interface output should stay at an "up/down" status.

Step 3   In enable mode at the router prompt enter the following (where 0 is the interface number):

clear int e0

Step 4   Check the interface status with show int e 0 (where 0 is the interface number).

Step 5   You should now see the interface status become "up/up", which indicates that the Cisco ethernet hardware is good.


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