How to configure PPPOA DSL on Cisco IOS Router

This topic is to discuss the following lesson:

Hi, René, first sorry for my broken English but I am a Spanish speaker, let me tell you that your explain are excellent about the dsl configuration, but I have a doubt I am trying to configure two router cisco 857 back to back another way to say that you know is stand alone, or it is necessary something clock signal from DSLAM to ADSL router, it is just an idea because here I have a case where two building have just two wire connection between them, what do you think is it possible or not, I was trying but it doesn´t work maybe you can help me .

Thank you a lot and greetings from Central America …

Hi Nelson,

As far as I know it’s only possible to configure back-to-back DSL using G.SHDSL, you can’t do it with ADSL because it’s asymmetric. G.SHDSL is symmetric.

If you have an ATM interface that supports G.SHDSL you can try this:

interface ATM0/0
dsl equipment-type ?

If it supports this command, you should be able to run back-to-back.


Hello Rene,

could you please draw the dsl connection on while using modem and while using router ?

how the network connection ?

Hi Gaganpal,

What do you mean exactly?


Hi Rene,

“we will configure a dialer interface that has all PPP settings”.
OK we are configuring it but can you please explain WHAT IS a “dialer interface”? A logical interface that DIALS somewhere (where?) something (what?)?
Or an interface in which someone dials into?
Thank you!


Hi Adrian,

There’s a separation between the “logical” configuration and the physical interface. The dialer interface is the logical interface where we configure the protocol.

In this example, we have an ATM physical interface and the protocol is PPP.


Hi Rene,

Thank you. I appreciate your very fast reply. I would appreciate if you could take a look to my explanation and tell if its ok and also answer some questions:

ok so interface dialer is a type of logical interface / a some kind of entity where we store some settings / paramteres. under this logical interface we configure parameters like “encapsulation ppp” … “ppp chap password SECRET”. This entity is attached to a pool (dialer pool 1).

interface dialer 1
dialer pool 1

In this pool we can include several physical interfaces as ATM :

interface ATM 1/0
dialer pool-member 1

interface fa0
dialer pool-member 1 ( can we do this?)

All physical interfaces member in this “pool 1” will use the logical interface named DIALER INTERFACE (also attaches to this pool) in order to reach somewhere ( to the ISP).
So Pool 1 has:
- one logical interface (DIALER INTERFACE) which contains all connection params (and also NAT), and
- one or several physical interface.

  1. can we include several physical interfaces to the same pool to use same dialer int. ?
  2. Is it ok ?
  3. Who initiates connection ? Always the CPE? sending the packets to dialer 0 interface because we have a def route to dialer 0?
  4. Why do we call it DIALER? does this interface actually DIALS like a code or something into the ISP net ( as we dial a phone number to the phone operator). I am trying to find a reason for its name…

Thank you again a lot Rene! You are REALLY helpful!!

Kind regards,

p.s.: you can take your time for the answer. I am not in a hurry,

Hi Adrian,

It sounds like you got it yes.

The dialer interface is the logical interface where we configure all protocol stuff. Things like PPP, CHAP/PAP authentication, etc.

A dialer interface can only belong to one dialer pool.

In the dialer pool, we can have one or more physical interfaces…physical interfaces can belong to one or more dialer pools. That’s totally cool if you want to use it.

When you use something like PPPoE, then it’s normally the CPE that “initiates” the connection. When the router has a packet with the dialer interface as the next hop, then this will trigger the dialer to become active. A default route is a good example yes.

It’s called a dialer because originally this was all used for analog modems / ISDN lines where we used dialing :slight_smile:

Hope this helps!


This seems to be no longer part of the CCNP Route exam topics, best I can tell as of today March 31st 2016.

Hi James,

You are right. I’ll remove it from the list.


Hi Rene/Moderators,

I am looking for a fuller understanding of the command “sh dsl interface atm 0”.
Do you have a post for this?
I searched dsl and arrived here…

Hello Ian

This command essentially displays information and statistics specific to the ADSL line.

You can find more information about this at the following Cisco link.

I hope this has been helpful!


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thanks, this link also helped

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Thank you all for sharing.
I need to understand why we configure encapsulation below PVC 0/42 please?
On what equipment we find this encapsulation at ISP network (on dslam or x…), Please?

Thank you,

Hello Brahim

The pvc command is used to enter into ATM virtual circuit configuration mode. In order to do so, you must specify the Virtual Path Identifier (VPI) and the Virtual Channel Identifier (VCI) in the format of VPI/VCI. These values together provide what is known as a connection identifier.

ATM is a connection-orientated protocol and as such, there is a connection identifier in every cell header which explicitly associates a cell with a given virtual channel on a physical link. The connection identifier is used in multiplexing, demultiplexing and switching a cell through the network. VCIs and VPIs are not addresses. They are explicitly assigned at each segment (link between ATM nodes) of a connection when a connection is established, and remain for the duration of the connection. Using the VCI/VPI the ATM layer can asynchronously interleave (multiplex) cells from multiple connections.

The choice of these values is up to the ISP and are typically configured within the DSLAM. The values used in the lesson were chosen randomly. More information on the specific command in Cisco devices can be found here:

I hope this has been helpful!


Hello Sir,

Thank you for your details.
My question is about :

Router(config-if)#pvc 0/42                        
Router(config-if-atm-vc)#encapsulation aal5mux ppp dialer

Could you tell me if it’s necessary to configure this encapsulation, and why please?
I can’t find it on your attachment.

Thank you,

Hello Brahim

When configuring the PVC circuit of a DSL connection, you must choose a method of encapsulation for what is known as the ATM adaptation layer or AAL. By default, the aal5snap encapsulation is set, but different types of encapsulation support different features. In particular, the aal5mux option used here specifies an AAL type for multiplexed VCs. When this is used, a protocol must be specified, and in this case, Rene is using ppp.

You can find out detailed information about the various options for these configurations at the following Cisco command-line reference documentation.

I hope this has been helpful!


Thank you so much Sir🤝

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Still a useful lesson even up just helped me resolve a DSL issue on Cisco 1921.thanks