CCNP - loopback

Author

loopback

rjohn

2001-05-24, 2:50 am

hi ppl,
bit confuse about loopback

My provider has test our E1 Connectivity with the help of loopback.

How is it possible, which command u can use to test the loopback? in 2610 Router or higher.

Cheers

Yankee

2001-05-24, 3:15 am

You can't "test" your loopback. This is hard to explain without a drawing but I'll try...

When you put up a loop it points away from you towards another device so that they can test to you.

Carriers frequently do this to test their circuits, but they most often do it to the NIU or to the CSU/DSU because they have the ability to put up that loop remotely and then knock it down when they are finished testing. On rare occassions they may ask you to put up a hard loop for them to test to, but that is uncommon.

Hope that helps,

Yankee

Dave Penner

2001-05-24, 6:36 am

quote:
Originally posted by Yankee
You can't "test" your loopback. This is hard to explain without a drawing but I'll try...

When you put up a loop it points away from you towards another device so that they can test to you.

Carriers frequently do this to test their circuits, but they most often do it to the NIU or to the CSU/DSU because they have the ability to put up that loop remotely and

Yankee

Very good expanation Yankee. Allow me to elaborate a bit further. The CSU/DSU unit at your end can be configured to send data back towards the device that sent it. This is the loopback. What happens is this. Either a remote test set... or one at the local CO is set up on your specific circuit. They send a data stream out that has the loop-up coding in it to activate the loopback on your CSU/DSU. Once this is completed they can do several tests. The most common is the BERT test. BERT stands for Bit Error Rate Test. Once the loopback is established the testing device sends out a data pattern. This pattern travels through the network and arrives at the CSU/DSU which sends it back towards the testing device. The device then checks the data pattern it recieves with the data pattern it has sent.

Cheers

Dave Penner

strikeattack

2001-05-24, 7:32 am

Don't forget about the lesser known ERNIE test. It is not as commonly used because it takes longer to do, and there is usually a client fee (expensive), because it requires WAN engineers on-site.

Expensive
Round-Trip
Node
Internetwork
Examintation

Dave Penner

2001-05-24, 7:34 am

quote:
Originally posted by strikeattack
Don't forget about the lesser known ERNIE test. It is not as commonly used because it takes longer to do, and there is usually a client fee.

Expensive
Round-Trip
Node
Internetwork
Examintation

Oh yeah... I failed to mention that one didn't I. You can also run the GROVER and BIG BIRD tests as well.

strikeattack

2001-05-24, 8:31 am

I'm sorry, I couldn't help it.

Dave Penner

2001-05-24, 8:38 am

quote:
Originally posted by strikeattack
I'm sorry, I couldn't help it.

No worries... I actually got a good laugh out of it.

Yankee

2001-05-24, 5:59 pm

Since we decided to get more advanced in our explanations I will add that the carrier can test at the T1 (or E1 as the question started) level as Mr Penner has described or they can run patterns at the channel level just covering the BW (channels)you are using.

Yankee

Trouble Man

2001-05-24, 7:21 pm

Also, the ERNIE test comes with a free rubber ducky. (Is it that easy to tell I got kids!!)

doctorcisco

2001-05-24, 11:17 pm

quote:
Originally posted by Yankee
Since we decided to get more advanced in our explanations I will add that the carrier can test at the T1 (or E1 as the question started) level as Mr Penner has described or they can run patterns at the channel level just covering the BW (channels)you are using.

Yankee

Just to add a bit to the discussion, the following pieces of equipment are loopable on a circuit:

1) The CSU/DSU.
2) The "Smartjack" (at the demarc). On a 56K line, telco may or may not have installed one; on a T1, they are supposed to be installed. Unfortunately, some smartjacks draw local power, and will not loop back during a power failure at your site.
3) Some flavors of repeaters in the local loop are loopable.
4) The OCU (Office Channel Unit) at the CO. Your carrier can run a loopback to the LEC's OCU to clear most of the LEC's network. The OCU might be a DS0DP on a 56K line.
5) Various MUXes and other equipment further back in LEC's network.

The OCU loop can be very helpful. If they can't loop it, or take errors on their patterns when they run to it, they can't blame your equipment, since they're bad to the last CO. When telco sees errors on a circuit to the CSU, they are *supposed to* "walk backwards" from there until the errors stop. They don't always do that. If they say they are taking errors to the CSU and want you to check cabling and equipment, ask if they took errors to the Smartjack (AKA an NIU) and/or the OCU.

However, there will be occasions when telco is good to the smartjack, bad to the CSU, and it's still their problem. Those get a bit ugly.

FWIW,
doctorcisco

P.S. Where do I sign up for MY rubber duckie?!?

Yankee

2001-05-25, 3:21 am

Don't forget the X-Jack at the extended DEMARC cuz it puts up a hard loop when the T1 cable is removed...

Yankee

kalpesh_vyas

2001-05-25, 12:06 pm

Hi Friends,

Well, I have implemented a nice big poject for DOT ( Department Of Telecommunication - India ) which involves Cisco 7507 router at central location and Cisco 2501s and Cisco 3640s at reomet site. We had 3 E1 lines ( 2 Mbps ) for 3 locations and let me tell u how we tested the same. I hope u might find the answer.

See, it is very important that what equipment do u use? Well, we used RAD HTU E1 for the connectivity for 2 Mbps E1 line. U need to install HTU E1 at each site ( for batter understanding consider it just like a modem ). On Cisco 7507, we used PA-MC-8 E1 ( 8 port channelised E1 Module ) and at another end we used Cisco 3640's fast serial port.

HTU E1 uses 2 pairs of cable for communication...., one is tx and the another is rx. It is also having built in feature of testing the loopback. By enabling that we can test the link from our local end router to local end HTU E1 ( By giving the command SHOW INTERFACE xxx - it will show that the serial port is up and line protocol is up and looped ). After testing the local link, we can ask the telecommunication department to test their like across the exchanges. In India they are having PCM departments where they have installed Multiplexers and used tu extend the lines between exchanges. I will not be able to explain u the operation of PCM / Multiplexer as it is very diff. without drawing / personal discuession.

In short I want u to understand that in case of E1, u cannot test ur line end to end, like in case of cable pair.

Exchange
Router - ASM20 --------------- ASM 20 -Router
cable pair
(64 kbps)

Above is an example of connecting 2 locations by using RAD ASM 20 (cable Pair modems). ASM 20 uses 2 pairs one is Tx and one is Rx. In the above case, if u reverse the Tx and Rx pair at one end, u will get the loopback at another end.

2 Mbps
PCM-------PCM
| E1 PRI |
Router - HTU E1--| |--HTU E1 - Router

In the above case, 2 Mbps E1 pri( chanellised ) is used to connect 2 locations by using HTU E1. Notice that in this case your cable goes from HTU E1 to the local exchange. From exchange it goes to Multiplexer and by using fiber the same is extended to the Multiplexer at another location. This is the reason why u can't test ur E1 line for the loopback like Cable pair...

I hope u can understand the above... I have tried my level best to make it simple...

By the way, to my all the friends and members, this is first time I am replying. If I have committed some mistake, pl. bare
with me.....

Lov / Kalpesh

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