PPPoE Help Please

Hello all.... Was wondering if someone could help me out with understanding PPPoE. I found a helpful article by H3C (google H3C pppoe and its the first link). I also tried looking up the frame format to understand the encapsulation.

What I don't understand it why a dsl modem doesn't send the frame toward the provider as ppp. I don't get why the dsl modem is taking a ppp frame and wrapping it in ethernet before sending it out. In the example I'm thinking about, there is a lan connected to a router. The router is connected to a dsl modem, which connects to a PPPoE server/DSLAM.

When the lan sends traffic to the router, the PCs are sending IP packets encapsulated in Ethernet. The router looks at these frames and must send it to the dsl modem. I assume that these frames are sent to the modem as plain ppp (so the router strips off the Ethernet header and replaces it with a ppp header, then hands it to the dsl modem). At this point the modem sends out traffic to the dslam as a ppp frame which is encapsulated in ethernet. Why not just send a ppp frame to the dslam?

I'm not sure what I'm missing..

Any help would be appreciated...

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Comments

tiersten

If it terminates the PPP session at the DSLAM/MSAN then it has to start doing routing and also coping with multiple providers.

up2thetime

So does that mean that the dsl modem sends it out as PPP?
I'm just not clear on where the ethernet encapsulation comes in, or how its useful...

This is my thinking....

Lan send ethernet frame to router

Router takes off ethernet header/trailer and encapsulated with PPP

Sends PPP frame to dsl modem. And dsl modem sends PPP frame to dslam.

Why is it necessary to encapsulate ppp in ethernet, and where does it happen?

This assumes the router at the customer is acting as the PPPoE client (not each pc on the lan).

vinbuck

Probably the easiest answer lies in the BRAS (Broadband Remote Access Server) which terminates the PPPoE session. Most DSL providers are going to authenticate their users one of two ways, either using some form of MAC Restricted DHCP or a PPPoE BRAS. When using a BRAS, PPPoE is the preferred choice for many DSL networks because once you get from the subscriber loop to the distribution and core layers, you are going to be mostly ethernet. PPPoE fits perfectly with this because it establishes a session over a bridged ethernet segment from the subscriber's DSL Modem to the DSLAM and then is usually native ethernet all the way back to the BRAS. The fact that the underlying physical transport between the DSL Modem and the DSLAM involves copper telephone cable is irrelevant because the DSL modem believes it is directly connected to the BRAS...imagine running a several thousand foot ethernet cable from the DSL Modem back to the BRAS and this is basically what ADSL and PPPoE accomplish.

Like Tiersten said...if you use PPP as your encapsulation protocal, you have start routing to get from PPP to ethernet. Remember routers connect dissimilar networks (like PPP and Ethernet - both Layer 2 technologies)

This diagram may help

Notice the host network is ethernet and the BRAS network (PPPoE Server in this diagram) is also ethernet, even though the networks in between aren't natively ethernet.

To address your original questions...

So does that mean that the dsl modem sends it out as PPP?
I'm just not clear on where the ethernet encapsulation comes in, or how its useful...

This is my thinking....

Lan send ethernet frame to router

In this case that's true...you are routing between a private ethernet segment to a public one with NAT translation.

Router takes off ethernet header/trailer and encapsulated with PPP

Since you are going from one ethernet network to another, PPP is actually encapsulated inside an Ethernet frame which is then encapsulated inside ATM for the duration of the DSL transport

Sends PPP frame to dsl modem. And dsl modem sends PPP frame to dslam.

DSL is an ATM technology, so the underlying Layer 2 encapsulation is actually ATM from the DSL modem to the DSLAM. A bridged ethernet segment is created, and PPP is encapsulated inside Ethernet (PPPoE).

Why is it necessary to encapsulate ppp in ethernet, and where does it happen?

Because PPP and Ethernet are two different Layer 2 encapsualtion protocols and normally require a router with a Layer 3 address to talk between them. PPPoE was specifically developed to allow PPP to travel over an ethernet segment by encapsulating PPP inside an ethernet frame.

This assumes the router at the customer is acting as the PPPoE client (not each pc on the lan).

Anything that is capable of PPPoE may be used as a client, a PC, Router, Firewall, etc.

up2thetime

Hey that was the same diagram i was using!

Okay, i think it's making a little more sense.

But considering what tiersten said, what is wrong with doing routing?

For example, the router can receive an Ethernet frame from hosts on the lan, then decide to send it to the DSLAM as plain PPP. The DSLAM could pick it up as PPP, rip off the PPP header and trailer, and then send to the BRAS as Ethernet. Why wouldn't this work?

I'm having real trouble with understanding why we are encapsulating twice at Layer2, and what the benefit is.... Is the BRAS picking up Ethernet frames, or PPP.........or both?

vinbuck

up2thetime wrote: »

Hey that was the same diagram i was using!

Okay, i think it's making a little more sense.

But considering what tiersten said, what is wrong with doing routing?

For example, the router can receive an Ethernet frame from hosts on the lan, then decide to send it to the DSLAM as plain PPP. The DSLAM could pick it up as PPP, rip off the PPP header and trailer, and then send to the BRAS as Ethernet. Why wouldn't this work?

I'm having real trouble with understanding why we are encapsulating twice at Layer2, and what the benefit is.... Is the BRAS picking up Ethernet frames, or PPP.........or both?

Because DSLAMS are designed to be layer-2 aggregators (ethernet switches do the same thing) and not routers. Most DSLAMS have minimal or no routing capability. They are designed to take several hundred DSL connections and funnel them all into one or two ethernet uplinks to the distribution or core layer. To sum it up, a DSLAM connects DSL Modems the same way a Layer 2 Ethernet switch connects end hosts. The BRAS will see an inbound session from the modem as ethernet and what you're asking about (PPP to Ethernet) is certainly technically possible, it's just not practical in a real world design.

up2thetime

MississippiGuardsman wrote: »

Because DSLAMS are designed to be layer-2 aggregators (ethernet switches do the same thing) and not routers.

Ahhh! That's where I was getting thrown off!

And the point of PPP is for the security/authentication and keeping track of individual sessions, which Ethernet by itself will not provide. Right?

vinbuck

up2thetime wrote: »

Ahhh! That's where I was getting thrown off!

And the point of PPP is for the security/authentication and keeping track of individual sessions, which Ethernet by itself will not provide. Right?

If you're referring to PPPoE then yes. Not to confuse the issue

, but I was working on a project this week where I had to bridge PPP to Ethernet (Multiple T1s to one Ethernet uplink) so that would be another way to get from PPP to Ethernet without routing, but I doubt there are many DSLAMS that have the command set to bridge ppp to ethernet....at least I haven't seen any but I mostly work on Adtran DSLAMS/MSAPS