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I have the following question, which I posted on the main site; however, it was closed as too broad.

  • There was a concern expressed since this is not a question about the operation of my own network, which is one of the on-topic conditions in the help center. Is the question below on-topic?
  • If the question is on-topic, what should I change to make it less broad?
  • What parts of the question need to be clarified?

Original Question:


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  1. How the router (home gateway router) decides when to use PPP? Means by looking at which parameters it decide ? is it only during VPN ?

  2. From where the role of L2TP starts ? is it from DSLAM ? My frnd told that it starts from BRAS/NAS. I think DSLAM could be a LAC.

  3. If a VPN is been established, then until the VPN terminates does all the data go through the L2TP ? Can we send data w/o tunnel during VPN connection ?

  4. During session establishment in L2TP there is a chance (is it a chance or always ?) that LNS can send a re-configuration message. If re-config happens for e.g if during starting PPP is established with PAP but LNS wants a CHAP then how that happens ? Because according to RFC host should not know about PPP and L2TP, so what type of request is been sent to the Host ? And also during the session if another LNS wants a PAP how it works ? Where the user authentication file saved in HOST side ?

  5. There is been mentioned that During re-configuration of L2TP by LNS it assigns private address to the LAC. So is this private address is something like new address to the HOST ? How this works ? How DHCP-S manages this ?

  6. After session establishment in L2TP, there is no role of LAC ? What about the Tunnel path ? Means does it come through BRAS or not , i think yes because of AAA ? But if tunnel comes through BRAS then how LAC avoids it ? how multiple sessions senario will work here if LAC avoids it ?

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I recommend you break your questions down into multiple questions which you will have to make several posts for. You have quesitions about how L2TP works for example that could be grouped together and questions about how the edge-to-core architecure of ADSL typically functions.

Firstly I recommend read about how L2TP works. What I have written below is just a loose suggestion, you have actually writen about 10 or 15 questions below. You could try making a couple of posts to Network Engineering asking just a couple of your questions. By learning the answers to a couple of your questions you will very likely understand the answers to some of the others. After that revise your questions, think about will you still don't know and then ask another question (by making another post).

These question are about the end user router configuration:

How the router (home gateway router) decides when to use PPP? Means by looking at which parameters it decide ?  is it only during VPN ?

If a VPN is been established, then until the VPN terminates does all the data go through the L2TP ? Can we send data w/o tunnel during VPN connection ?

These are all questions on the fundamental workings of L2TP:

From where the role of L2TP starts ? is it from DSLAM ?
My friend told that it starts from BRAS/NAS. I think DSLAM could be a LAC.

After session establishment in L2TP, there is no role of LAC ? What about the Tunnel path ? Means does it come through BRAS or not, i think yes because of AAA ? But if tunnel comes through BRAS then how LAC avoids it ? how multiple sessions senario will work here if LAC avoids it ?

During session establishment in L2TP there is a chance (is it a chance or always ?) that LNS can send a re-configuration message. If re-config happens for e.g if during starting PPP is established with PAP but LNS wants a CHAP then how that happens ? 

Because according to RFC host should not know about PPP and L2TP, so what type of request is been sent to the Host ?

And also during the session if another LNS wants a PAP how it works ? Where the user authentication file saved in HOST side ?
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  • thanks for the advice i will do that, why i posted all the question at a time so that somebody can answer in a specific way if i am posting at different times then i may not get to the point – dillip_beta Aug 27 '14 at 6:03

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