Atm, PPPoE and PPPoA

kpjungle · December 2008

Hi,

I am studying up on the different PPPoX stuff, and i find it confusing why you dont use PPPoE when the service-provider interface is Ethernet, and PPPoA when the interface is ATM. Why the PPPoE on an ATM interface?

As I would understand it, you would use encapsulate PPP into an ethernet frame on an ethernet interface (PPPoE). You would encapsulate PPP into an ATM cell on an ATM interface (PPPoA)... but alas, the PPPoE on an ATM is escaping me.

I think im missing some key-piece here to understand that part.

If someone could shed some light on that, more than what has already been mentioned on the forums, I would appreciate it. Thanks!

WRKNonCCNP · December 2008

I had a similar problem when i was studying for the ISCW (need to update sig). PPPoE with an Ethernet interface encapsulates the PPP PDU into an Ethernet frame which is transported out the Ethernet interface toward the service provider. Simple enough.

PPPoE with an outbound ATM interface still encapsulates the PPP PDU in an Ethernet frame, but that Ethernet frame (and PPP payload) is transported in the payload of ATM cells. Think of the ATM portion of this connection kinda like a tunnel. The ATM cells are transported using the pvc (vpi/vci) to the DSLAM or Aggregation router, where ever the ATM pvc terminates. Once there, the ATM headers are removed, the ATM cells are reassembled, and you have PPP encapsulated in Ethernet again, where it is dealt with accordingly. I think PPPoE with an ATM interface just allows you to use an ATM network to pass your PPPoE frames between the CPE and whichever device terminates the ATM pvc.

Hope that helps. The cisco exam cert guide didn't do too good a job of explaining that stuff IMO.

lildeezul · December 2008

so is PPPoe on atm interface like PPPoAoE ?

WRKNonCCNP · December 2008

Yes, PPP within an Ethernet frame, divided up into ATM cells for transport

redwarrior · December 2008

Bleh...I'd have to say this is my least favorite part of ISCW...can't wait to get this one done and get on to switching!

lildeezul · December 2008

Aww man.. This is my favorite part.

configure DSl on the Atm interface, using the pppoe-client dial-pool-number ..... and using a dialer interface and configuring ip address negotiated, and ppp authentication chap/pap callin
ppp chap sent username xxxx yyyy..... ip mtu 1492... ip nat out...

man this is fun,..

redwarrior · December 2008

I must say, lildeezul, that you may be a sick and twisted individual...the moment I saw ATM was involved, I felt ill...ATM? Wha? Didn't that go out when grandpa quit networking? LOL...at least someone has some love for ATM.

kpjungle · December 2008

I guess im a bit at a loss on the physical transport used.

When reading the material, it would seem the physical transport from the CO (DSLAM) to your house (ADSL modem) is ATM, is that correct?

Then you would only use PPPoE to get the data encapsulated over an ethernet frame to the dsl modem, will this dsl modem then strip the ethernet, and encapsulate with ATM?

lildeezul · December 2008

correct me if i am wrong anyone, becuase i too am learning about this technology.

the physical access would be the rj-11 cable from your wall jack/local loop to that dslam... the layer 2 transport might be ATM....

but i believe ATM is used between the DSLAM and the ISP edge router. so really i belive this would make the connection PPPoEoA... becuase you using ppp encapsulated into a ethernet frame, then broken down into cells to be transport to over the ATM to the ISP.

if i were to used PPPoA on the ATM interface, and the layer 2 transport between the DSLAM and the ISP's edge router was using ethernet, then the encaspulation would really be, PPPoAoE.. .becuase your taking ppp and encapsulated them into ATM cells, which is then ecapsulated into a Etherframe to be used to be transported over the ethernet layer 2 transport from the DSLAM to the ISP.

so if the ISP is using ATM to connect to the dslam, then the last two letters would be 'oA'... and if i was using pppoe, then the final encapsulation would be PPPoEoA....

so really i guesss it depends on what the ISP is using to connect their router to the dslam.

again, someone correct me if I am wrong, but i think this is how it works. !

kpjungle · December 2008

lildeezul wrote:

correct me if i am wrong anyone, becuase i too am learning about this technology.

the physical access would be the rj-11 cable from your wall jack/local loop to that dslam... the layer 2 transport might be ATM....

but i believe ATM is used between the DSLAM and the ISP edge router. so really i belive this would make the connection PPPoEoA... becuase you using ppp encapsulated into a ethernet frame, then broken down into cells to be transport to over the ATM to the ISP.

if i were to used PPPoA on the ATM interface, and the layer 2 transport between the DSLAM and the ISP's edge router was using ethernet, then the encaspulation would really be, PPPoAoE.. .becuase your taking ppp and encapsulated them into ATM cells, which is then ecapsulated into a Etherframe to be used to be transported over the ethernet layer 2 transport from the DSLAM to the ISP.

so if the ISP is using ATM to connect to the dslam, then the last two letters would be 'oA'... and if i was using pppoe, then the final encapsulation would be PPPoEoA....

so really i guesss it depends on what the ISP is using to connect their router to the dslam.

again, someone correct me if I am wrong, but i think this is how it works. !

Im not sure about either

.. I am just wondering what the datalink technology used on the local loop (entry into house) toward the DSL is, cause that must be what we encapsulate something into.

lildeezul · December 2008

Ahhh... i see what you mean...

yeah i really havent though about from the local loop to the Dslam..

man... kpjungle.. all this ppp is confusing huh ?

anyone care to help us

:

WRKNonCCNP · December 2008

DSL provides layer 1 between the DSL modem (or router) and the DSLAM. Remember the physical layer can be broken down into two sublayers, the Transmission Convergence sublayer and the Physical Medium Dependent sublayer. The TC sublayer puts bits in order based on the protocol being used (framing) and the PMD transmits the bits across the medium (line coding) (copper in the case of DSL).

If you look at the ISCW official exam cert guide on page 94, you will see they list three different ways that data is transported from the CPE to the Aggregation router: RFC 1483/2684 Bridging, PPPoE, and PPPoA.

RFC 1483/2648 bridging is what we discussed as PPPoEoA, basically bridging pppoe frames into atm cells for transport, so pppoe with an outgoing ATM interface.

PPPoE is ppp encapsulated into ethernet and sent out an ethernet interface. Say your pppoe router was connected to an external DSL modem via ethernet. The ethernet frames leaving your router on the cat 5e cable or whatever are modulated by the modem to travel across the DSL link to the DSLAM.

PPPoA is ppp encapsulated directly into ATM cells, without the ethernet framing used when doing RFC 1483/2648 bridging. DSL is then just the physical transport and the ATM VC determines how the aggregation router is reached.

To be honest, i don't have the clearest understanding of how it all works (i spent the least time on this for the ISCW). Although this discussion has allowed me to further clarify my understanding. I believe that the above information is correct, and it is consistent with the information i have read, but i am by no means an expert. So take it as my $.02 on this topic.

lildeezul · December 2008

Well, i am finally getting started on my reading. hope to get a grasp on this soon

lildeezul · December 2008

Well i just got down with Part 1 of the ISCW book, (DSL/Cable part)

and WrkNonCCNP, you pretty much had it down pact.

i took good notes on this chapter.

i must include in your explanation, According to the book, ATM is the backbone that data in tranported through to reach the aggregation router. So basically using PPPoE on an ATM interfaces creates extra overhead, becuase it all has to travel through the ATM backbone. so every connection technically has an "oA" on the end on it.

when using PPPoe on an ethernet interface, connected to your dsl modem, which goes to the local loop. DSL is the layer 1 technology to reach the DSLAM, layer 2 doesnt happen until it reaches the ATM backbone, and layer 3 gets negotiated once the ppp session has been open. The dsl modem using PPP's LCP, gets the ATM PVC from the ISP.. (even though you dont configure it, becuase its the dsl modem not the cisco router)

thats why when you use an ATM interface (wic-1adsl) you have to specify the PVC (VPI/VCI) numbers.

this was some pretty cool chapters, learning about two technologies.. It made me put my thinking cap on.

but basically when i read part 1, and then refered back the WrkNonCCNp's post, there were complemtary. So thanks WrkNonCCNP

WRKNonCCNP · December 2008

glad i could help. discussions like this are useful to everyone who is involved, so thanks kpjungle for starting the thread and lildeezul for your input as well.

lildeezul · December 2008

learned something new today, when taking more notes, and re-reading part 1 of the ISCW exam cert guide, just to make sure i really know my dsl technology.

anyways... you know how we should put the MTU of ethernet interfaces if running PPPoE to 1492, becuase, etherenet will try to shove 1492 bytes of data into a frame, and then the rest of the 8 bytes will be padded with PPP headers and PID.

anyways, when running PPPoE on a ATM interface, then doing the math, and how ATM tries to make 48 byte payloads, if you set the MTU to 1454, then you can achieve a .6% increase of effiecient and overhead loss... this doesnt sound much but everything helps.

Will post my notes on here later, so everyone can view, and hopefully it will help people understand .

redwarrior · December 2008

When it comes to PPPoE and PPPoA, I don't think there is any such thing as too much info...the terminology can get pretty darn confusing.

I've configured PPPoE in a production environment on ASA 5505's, but this is a whole different matter.

kpjungle · December 2008

My observations:

PPPoA = Used when your router, is directly equipped with an ATM interface, and no DSL modem in front of it, in which case you get to set the encapsulation to AAL5mux (PPPoA routing).

PPPoE (On ATM), you are using an ATM interface, but probably have a DSL modem in front, doing the "real" ATM instead of you.

PPPoE (On ethernet), you just have a standard ethernet connection to your DSL modem, which does all the encapsulation for you.

All of these technologies, rely on ATM as L2 techology from the DSL modem (or if you dont have one, your router (PPPoA)), and L1 will be the DSL (Linecoding being G.Lite, CAP or DMT).

Those are just what my brain thinks at the moment, please correct if im wrong

gorebrush · December 2008

Pretty sure my 877 Router has a DSL modem to connect to the internet...

PPPoA is used there, too.

kpjungle · December 2008

gorebrush wrote:

Pretty sure my 877 Router has a DSL modem to connect to the internet...

PPPoA is used there, too.

The 877 then has an ATM interface?, and is anything in "front" of it?

_maurice · January 2009

lildeezul wrote: »

learned something new today, when taking more notes, and re-reading part 1 of the ISCW exam cert guide, just to make sure i really know my dsl technology.

anyways... you know how we should put the MTU of ethernet interfaces if running PPPoE to 1492, becuase, etherenet will try to shove 1492 bytes of data into a frame, and then the rest of the 8 bytes will be padded with PPP headers and PID.

anyways, when running PPPoE on a ATM interface, then doing the math, and how ATM tries to make 48 byte payloads, if you set the MTU to 1454, then you can achieve a .6% increase of effiecient and overhead loss... this doesnt sound much but everything helps.

Will post my notes on here later, so everyone can view, and hopefully it will help people understand .

Check out my thread here: http://techexams.net/forums/showthread.php?t=36487

Even if the MTU is set at 1454, the actual MTU used will be 1452. I ran a packet capture and tested this. 1454 reduced by 20 (the IP header length) and divided by 8 is not an absolute number. It needs to be an absolute number because the IP fragmentation offset in the IP header is multiplied by 8.

For example:
(1500-20)/8 is 185. 185 will go into the fragmentation offset value. (1454-20)/8 is 179.25.

It is hard to explain, but it is cause the fragmentation offset value is multiplied by 8. A cisco will accept most any number as the MTU value, but it always chooses the lower value that can go into the fragmentation offset field.

Atm, PPPoE and PPPoA

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