In loopback, the serial port sees its own keepalives come back. They are sequenced so it can tell. The MySeq number would be going up, but the MineSeen number wouldn't since nobody else is seeing them. I think that's the gist of it. There's a lot of info about this on Cisco's site. There's one page that says something about it randomizing the sequence number so it can be sure it's in loopback, but I can't find that now. That might be when you put manually put it in loopback, anyway.
_______________________________ Priscilla Oppenheimer www.troubleshootingnetworks.com www.priscilla.com Cable Guy wrote: > > 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=57030&t=56973 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
