ADSL configuration.

lildeezul · December 2008

Hello everyone, i am just reading some cisco documentation to get a headstart on my ISCW studies..

I was looking at configuration for Cisco ADSL config... and wanted some clarification to see if I am understanding this correctly.

ok... When using the "ISP provided dsl modem" connected to the ethernet port of a cisco router. you would use PPPoE correct ?

2) i see with the Cisco wic-1ADSL, the interface is called ATM0/0.... This is an ATM port ?? and with this interface you can configure PPPoE or PPPoA... the difference is the encapsulation command under the ATM interface... (encapsulation aal5mux dialer)?

what weird is that even though it is an ATM port it can still use PPPoE.. it would seem that it would use only PPPoA, because its an ATM port.. but the only "real" difference is that encaspulation command under the ATM port that tells it to route.

3) the actual physical port doesnt get the ip address, or isn't specified by IP NAT OUT. the only thing you do under here is use the enable pppoe , or use DSL auto config mode for an ATM interface, and the other thing is associate it with the dialer interface.

4) the dialer interface actually uses the IP negotiated command, and encapsulation ppp, along with the CHAP information.

Do i seem to be understanding this???

But what gets me, is how it actually looks....

With a modular router (2600,3600) they make WIC-1ADSL cards, which are ATM interfaces and the phone line comming from the JACK goes directly into the WIC card....

like this

but how is it hooked up with the 877 SOHO routers... They have a WAN port that is an RJ45...
how is that hooked up to the DSL line?? In Cbt nuggets jeremy's config showed the outside interface being an Ethernet port 4... Was this port connected to the DSL modem or something ?
I believe that the only way you could use the WAN RJ45 port of the SOHO routers is by connected it to an external DSL modem.

like this

telephone jack

DSL MODEM

(ether4) SOHO 877/871 ???

is that how they are hooked up???

NOTE***::::: excuse my ignorance, i am new to this technology on cisco devices, and i am just trying to understand as much as possible...

again thanks

lildeezul · December 2008

Ok i went back and read up on this thread
http://techexams.net/forums/viewtopic.php?t=22006

this had very good information posted by keenon and mgeorge.

basically what i picked up was the following:

remember, how i told yall that jeremy used ether 4 as his outside interface to the dsl telco.

well mgeorge simply put that if your router doesnt have a WIC-1adsl, that you can take e0/0 off and plug it into the dsl modem (netgear/dlink ect.) and then associate the Ethernet port to a dialer interface.

i have one question about this ^^^ if you plug into the dsl modem (netgear) wouldnt the dsl modem automatically be doing the negotiation and the establishment of the ppp circuit/session, and if you config the router to do it.. wouldnt it be doing it like twice a kinda repeated ???

also looking at Georges post, he said he has the WIC-1ADSL, and looking at keenon's post, that is the ATM interface, and the ATM interface is associated with the dialer pool which is associated to the dialer interface.

did anyone else interpret this like or did.... or did anyone interpret this different ?

it just seems weird that even though your using an ATM interface ( WIC-1ADSL) your still configuring PPPoE , and not PPPoA ... can anyone help explain ?

dtlokee · December 2008

The 877 has a built in ADSL interface that you plug the telephone line into (basically an integrated wic-1adsl card). When you configure this you will use PPPoA since the DSL circuit is terminated on the router. If there was a modem in front of the router and a Ethernet connection between the router and the modem, then its PPPoE

lildeezul · December 2008

Awww.. thanks that clears up my confusion about When to use PPPoe v PPPoA....

isnt it that if you use PPPoE the ppp stack goes back to the PC's, becuase they have to support and install pppoe software. and when using PPPoE the session is actually done at the routers port, not with the pc, so when using PPPoA you dont have to install software on the pc's ? Is that correct ?

Also is their an advantage using one over the other? or does it just depend on how you terminate and are hooked up to the local loop ?

whats more common? probably enterprises using DSL are using PPPoA, and small homes use PPPoE more often huh , becuase of the ease of installing the dsl modem provided by the ISP?

dtlokee · December 2008

Much of the issue stems from the fact that Ethernet and ATM don't support authentication which is a good thing to have with a "dial up" type service. The reason why PPP is used it to provide that extra layer for authentication. I use the 877 for all of my remote users that have DSL connections, they have been rock solid along with DMVPN configuration for spoke to spoke IPT calling. I wouldn't use a DSL modem in a business environment, they are way to flaky and I prefer to terminate right into the router with an ADSL connection.

Bleego · December 2008

lildeezul wrote:

well mgeorge simply put that if your router doesnt have a WIC-1adsl, that you can take e0/0 off and plug it into the dsl modem (netgear/dlink ect.) and then associate the Ethernet port to a dialer interface.

i have one question about this ^^^ if you plug into the dsl modem (netgear) wouldnt the dsl modem automatically be doing the negotiation and the establishment of the ppp circuit/session, and if you config the router to do it.. wouldnt it be doing it like twice a kinda repeated ???

When PPPoE was first being implemented by ISP's the authentication was done on software that was installed on your PC and then you would plug into the modem ( which was in bridge mode ) to connect to the DSL service. Naturally people hated this, and ISP's started providing authentication services on the modem. If you were using one of these modems, and you wanted the Cisco to handle the authentication you would need to disable that functionality on your modem and configure it for bridge mode.

lildeezul wrote:

it just seems weird that even though your using an ATM interface ( WIC-1ADSL) your still configuring PPPoE , and not PPPoA ... can anyone help explain ?

( I will preface this by saying that this is mostly from working with SBC / AT&T DSL offerings, information may vary by providors, It has been a Looong time since I have worked with DSL so if Im off, somebody feel free to smack me with a frying pan. )

ADSL Networks are usually ATM on Layer 2, this is why ADSL Wic cards are configured as ATM interfaces. PPPoE and PPPoA is all determined by the ISP. For a while SBC had both PPPoE and PPPoA for DSL ( before that it was all bridged static IP's ) PPPoE circuits had a bridged modem, and PPPoE authentication was handled by the PC.

PPPoA had things like USB modems and Internal DSL cards, and then, again, software for authentication on PC's. PPPoA was dropped pretty shortly at least in SBCland. The one instance I have seen where the customer had their own ADSL setup ( from remote user - Telco Layer 2 network - Corporate Office ) They did not use PPPoE or PPPoA, it was just setup as a point to point subinterface.

It has been a long time so I don't remember the exact specifics between them anymore. http://en.wikipedia.org/wiki/Point-to-Point_Protocol_over_ATM and the linked PPPoE page are a good place to start. Sorry for being all over the place there, but I guess the short version is that the type of interface does not necessarily have anything to do with whether you are using PPPoE or PPPoA. You will most likely see PPPoE in the US.

redwarrior · December 2008

You can terminate PPP wherever you like. In most cases, I like to terminate PPP on either a router or, better, a firewall and then NAT clients behind the firewall using PAT to the public IP. This saves each client having to be configured to authenticate in order to use the connection and I like to keep as much of the configuration on my end as I can. I haven't seen PPPoA in the wild yet, but then, whenever I deal with DSL it's generally just an internet connection for a remote office that I then use an ASA to build a VPN connection over, so we don't have a router connecting directly to a line, but an ASA connecting to the provider's DSL device. I would think that the routers with the DSL cards would be handy as a backup connection for a site with a beefier main connection.

ADSL configuration.

Comments