Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 
Community Member

Layer 1 Loop test over MPLS

I was asked lately if it is possible to run a layer one loop test on a T1 through an MPLS cloud. My answer was that it does not matter. MPLS is at a different layer. I was told that this is different. When you send a loop code, when testing a T1 with ESF, it is not possible.

Here we have two sites having point to point connection (Pure layer 1 ) and going through an MPLS cloud.


Re: Layer 1 Loop test over MPLS


i feel on both the locations you have your sites connected to the local TELCO which in turn connects them up to the SP network or to the backbone.

now you get that looped (remote loop) at site A TELCO side towards site B and wanna check at site B if this is the case then i feel there shouldnt be any problem in seeing the loop from TELCO.

we did conduct this kinda excersie in the past while troubleshooting some of our links which were just fine..


Community Member

Re: Layer 1 Loop test over MPLS

Hi there,

Thank you for your prompt reply.

The question that through the discussion was that I want to run a pure hear to head testing from the Smart Jack (HRU) of site A to the remote HRU of site B. The two sites are connected in point-to-point manner but through an MPLS topology.

Can we inject a loop code from site A that goes all the way to site B? and see if there is a loop on the remote site. This is a purely Layer 1 test that involve signaling. No layer 2 in the picture.

In my opinion, we cannot. Because we would have two Telcos plus the whole MPLS network in the middle. We can run two loop test at each part of the link.

I wanted just to know if the signaling at the layer 1 will also be labeled and sent to the remote or not.


Re: Layer 1 Loop test over MPLS

I really dont understand your issue here! You say a T1 through an MPLS cloud? Is this using ATOM to provide this service? If so then a loop test might have some difficulty with certain bits being stripped at the end point due to the configuration from your SP. If its just normal SDH (Sonet) then it should just act like normal.

Community Member

Re: Layer 1 Loop test over MPLS


OK! I am going to try to be clear about this.

It is not really an issue. This was a discussion that took place about a two sites (A and B) that are connected in a point-to-point with a T1.

Now this connection is being moved to an MPLS cloud.

The tech was asking, in the case he wants to run a loop test at the physical layer. How should he proceed? I told him that he has now two local loops, he needs to test both separately. He replied that he wants to test all the way through the MPLS cloud to reach the remote site. He said that when sending a loop code with ESF, the PE strips the ESF part from the loop code and label only the payload (which is not clear to me).

Imagine you have a test device connected to the Demarc A and you are testing the link up to Demarc B. Can you run the loop test across the MPLS cloud?


Re: Layer 1 Loop test over MPLS

ah ok so you have two sites both of which terminate on an mpls cloud. Your engineer wants to go to site A and loop site B. Nada, not a chance, no, no way hose and all that. Both your T1 circuits as you say are terminated on say Cisco equipment within a channelised card. This is where your ESF and all sonet/sdh bits are terminated. From there on in you on pure mpls/ip. The only way to test these circuits is as you already stated - go to site A and get a loop put on at the local PE end - that then lets you ensure that circuit A is fine. repeat for circuit B. Your engineer has to remember that mpls isnt an SDH/sonet cloud.


CreatePlease to create content