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Home > Archive > CCNP > February 2002 > Some what at a loss re: LoopBack
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Some what at a loss re: LoopBack
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| Yeti-GBR1 2002-02-03, 5:44 pm |
| Can anyone explain when and why (apart from troubleshooting) that the loopback command may be of use?
I have just read the following article from Cisco : http://www.cisco.com/warp/customer/471/73.html#command
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loopback line
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Syntax
[no] loopback line [payload]
Description
The loopback line interface configuration command puts the CSU/DSU module into line loopback. There are two type of line loopbacks. Without the payload parameter, the line is looped through the CSU portion of the module. In payload loopback, the line is looped through the DSU portion of the module.
56k CSU/DSU Module:
When the 56k CSU/DSU module is placed into loopback line, the CSU/DSU module loops the line through the CSU portion of the module and loops the DTE interface back to the router. The Adtran terminology for this loopback is "DTE and loop." If the CSU/DSU is configured for switched mode, there must be a connection established in order to do a line loopback.
56k CSU/DSU loopback line
When the 56k CSU/DSU module is placed into loopback line payload, the CSU/DSU module loops the line through the DSU portion of the module. The Adtran terminology for this loopback is "loop only." If the CSU/DSU is configured for switched mode, there must be a connection established in order to do a line loopback.
56k CSU/DSU loopback line payload
T1 CSU/DSU Module:
When the T1 CSU/DSU module is placed into line loopback, the CSU/DSU does a full bandwidth loopback through the CSU portion of the module. It regenerates the signal back to the line.
T1 CSU/DSU loopback line
When the T1 CSU/DSU module is placed into line loopback payload, the CSU/DSU does a loopback through the DSU portion of the module. Data is only looped back on the configured timeslots. The line loopback payload command reframes the data link, regenerates the signal, and corrects Bi-Polar Violations (BPV) and Extended Super Frame (ESF) CRC errors.
T1 CSU/DSU loopback line payload
Application
This command applies to the two-wire 56k, four-wire 56k, and T1 CSU/DSUs.
Default
no loopback line
Example
interface serial 0
loopback line
and I'm at some what of a loss as to why/when I would use it  . | |
| haseeb_eng 2002-02-04, 12:30 am |
| A loopback interface will work as a logical interface which never goes down even if the physical interface goes down then also it will keep on working because this is a logical interface not physical .
May be other members can explain better then me | |
| Yankee 2002-02-04, 4:04 am |
| That is not the loopback interface that Yeti asked about.
He is refering to a troubleshooting tool that I believe can only put up a loop in one direction (towards the router), so it isn't that useful. Basically it helps you test a small portion of your internal hardware and that's it.
Yankee | |
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Hello Yeti
I checked this on the Cisco site, and from what I can determine, there are two types of loopback tests. One which tests to a digital loop at the CSU part of the CSU/DSU - to test the TX/RX of the router interface, and the other test loops at the DSU part of the CSU/DSU to test the functionallity of the CSU/DSU itself. There is another option; a remote loopback test, which loops the distant end CSU/DSU to test the leased line as well as both DCE devices and the router interface.
The closest analogy I can come up with, particularly in Europe, is that of the V.54 and V.54bis modem loopback commands.
Hope this link sheds some more light on the subject; scroll down to 'loopback line'.
http://www.cisco.com/univercd/cc/td...#xtocid17242178
Cheers
Hippo | |
| haseeb_eng 2002-02-04, 4:47 am |
| well i did not read the whole post i just read the first line and reply him back . | |
| Yeti-GBR1 2002-02-04, 5:40 am |
| Cheers lads, that clears it all up...as I suspected its mainly for troubleshooting and thats it.
haseeb_eng re: "I did not read the whole post i just read the first line", may be you should read the whole question next time it kinda helps you know.  |
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