I've been working across many sites around the world with ADSL. The first output below is the one I've used many many times & I'm therefore most familiar with. It's from a site in the UK.

interface ATM0
 no ip address
 no atm ilmi-keepalive
 pvc 0/38 
  encapsulation aal5mux ppp dialer
  dialer pool-member 1
interface Dialer1
 ip address negotiated
 encapsulation ppp
 dialer pool 1
 dialer-group 1
 ppp chap hostname xxxxxx
 ppp chap password xxxxxx
 ppp authentication chap callin

I understand how this works. Now the stuff I don't understand. This config below has been taken from a site in Turin in Italy.

interface ATM0/1/0
 no ip address
 ip mtu 1492
 atm bandwidth dynamic
 no atm ilmi-keepalive
interface ATM0/1/0.1 point-to-point
 ip address xxxxx xxxxxx
 ip mtu 1492
 pvc 8/35 
  vbr-nrt 896 896 1
  encapsulation aal5snap
  max-reserved-bandwidth 97

This is the only stuff in the config to get it working. There is no dialer, no ppp username or password. This is literally all the config (except obviously a default route) required to get it working. So please may someone explain to me how this works?

The way I've been taught is that you get your sync (basically a physically good network to the ISP, i.e. layer 1 connectivity). Next you get a session, which is where you create your PPP session to the ISP, who authenticates you (this is layer2). Finally, at layer 3 you can then get to the internet (i.e. beyond the ISP's network). So how does this aal5snap work, and how does this config work to get them to the internet?

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer. – Ron Maupin Jan 3 at 4:25

You are familiar with IP over PPP over ATM.

The config you don't understand is IP over ATM, that is the ATM frames are directly transporting IP frames.

ATM was designed to carry many different protocols using many different encoding methods.

One reason some operators (mostly central Europe) choose to add PPP in to the mix is because it allows them to offer access to other operators relatively easily, i.e. the PPP sessions would actually terminate with another operator not where your physical connection is.

3rd common way to do this is IP over Ethernet over ATM, which is yet another config.


Take a look at RFC1483. Basically, this is IP over ATM. You can either don't do authentication and assume that anyone connected to your BRAS is a valid user, or use something like DHCP for assigning IP's based on specific client attributes.


This is IP over ATM (no PPP). You don't need authentication (and PPP) there. The "user" is specified by the pvc (8/35) in the config. Every line has their own PVC which consists of the ATM VPI/VCI.

For more information there is a FAQ from Cisco for ATM PVC.

The PVC has to be the same on both ends and is provided by the ATM provider.

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