Yes, that describes the loopback plug. But, loopback isn't just a crossing of the wires, but a state the router is in as well. If there is a fault in the wiring, or csu/dsu is put in loopback mode, the router will see and change to "loopback mode" which you can see in show controllers. Why? What is in the digital signal to tell it this? Something in normal traffic packets, or is it keepalives, anything else? If it is just keepalives, and you turn them off, would normal traffic ever trigger router to see the loopback? I don't have a T1 to make easy rj-45 loopback, and I hesitate to cut open my db60 serial cables.
If someone has a t1 card, and wants to try this, here is instruction to make loopback plug. Do a debug and see what happens. http://www.cisco.com/univercd/cc/td/doc/cisintwk/itg_v1/tr1915.htm#xtocid56 Step 1 Use wire cutters to cut a working RJ-45 or RJ-48 cable so that there are 5 inches of cable and the connector attached to it. Step 2 Strip the wires. Step 3 Twist the wires from pins 1 and 4 together. Step 4 Twist the wires from pins 2 and 5 together. Leave the rest of the wires alone. The pins on an RJ-45/48 jack are numbered from 1 through 8. Pin 1 is the left-most pin when looking at the jack with the metal pins facing you. Performing the Loopback Plug Test -------------------------------------------------------------------------------- Step 1 Insert the plug into the T1 port in question. Step 2 Save your router configuration using the write memory command. maui-nas-03#write memory Building configuration... [OK] Step 3 Set the encapsulation to HDLC. maui-nas-03#config terminal Enter configuration commands, one per line. End with CNTL/Z. maui-nas-03(config)#interface serial 0 maui-nas-03(config-if)#enc maui-nas-03(config-if)#encapsulation HDLC maui-nas-03(config-if)#^Z Step 4 Use the show running-config command to check whether the interface has an IP address. If the interface does not have an IP address, obtain a unique address and assign it to the interface with a subnet mask of 255.255.255.0 maui-nas-03(config)#ip address 172.22.53.1 255.255.255.0 Step 5 Clear the interface counters using the clear counters command. maui-nas-03#clear counters Clear "show interface" counters on all interfaces [confirm] maui-nas-03# Step 6 Perform the extended ping test as described in the "Using Extended ping Tests" section, earlier in this chapter. >From: Mary.KVITASHVILI >To: [EMAIL PROTECTED] >Subject: RE: why loopback plug detected as loopback in first place? >[7:56883] >Date: Tue, 5 Nov 2002 14:08:10 +0100 > >If I am not mistaken, basically in the wiring, the transmit line is crossed >to the receive so that signals going out are looped back to the receive. > > When you put a hardware loopback plug on a serial to test it, why does >the > > router see it as a loopback? With the show int serial command, you will >see > > -----serial up, line protocol up (loopback mode)----- > > Of course it needs to see it as loopback otherwise the router wouldn't > > operate and allow loopback tests. But what part of the digital signals > > specifically tell the router that what it sent out has just come back? >It > > can't be the carrier detect because that could be interpreted as a >carrier > > detect from any csu/dsu. Maybe a timing issue on the return signal, but > > sounds like the keepalives are being interpreted here somehow? Someone > > know the details of this? Help. _________________________________________________________________ Protect your PC - get McAfee.com VirusScan Online http://clinic.mcafee.com/clinic/ibuy/campaign.asp?cid=3963 Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=56973&t=56973 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
