Here is a picture of a an xDSL deployment : enter image description here

The BRAS is the termination for PPP sessions. The customer modem establishes a session with the BRAS, gets authenticated and can then access the ISP's network and Internet.

If I understand correctly, it is at the BRAS that the PPPoE and PPP headers will be stripped off the packet.

1) Are those headers added to packets coming from the Internet to the client before getting pushed to the DSLAM - modem, or are they only one way?

2) If they are added both ways, how does the BRAS know which session it is? Is it based on the client IP?

3) Do BRAS keep a session table, like a firewall would for an ACL or NAT for packet returns?

In short, I just want to know if the BRAS adds PPP/PPPoE headers, and doesn't just strip them off packets.

  • 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 could provide and accept your own answer. – Ron Maupin Aug 15 '17 at 5:38

Yes - the BRAS is both stripping the PPP[oE] frame (...and some L2/L3 underlay information, depending on the design in use) off of frames coming from the subscriber device and is adding both the PPP[oE] header and associated L2/L3 mapping information to get packets from the Internet back to the subscriber.

Part of the process of the subscriber device logging in to the BRAS is establishing a mapping between the subscriber's assigned IP and the necessary underlay information required to get inbound packets encapsulated and on their way to the edge. This is, in fact, a session table and it's one of the reasons why BRAS are fairly specialized devices (read: lots of state + high traffic volume).

  • What do you mean with "some L2/L3 informations"? What informations? I believe the AAL5 headers of xDSL are removed by the DSLAM. – pHeoz Dec 2 '16 at 10:28
  • There's some variation in mechanisms depending on the generation of the networking equipment and the type of termination. A given DSLAM might drop off an ATM circuit and map connections for a given BRAS to a VPI. In other situations the DSLAM might produce native Ethernet that gets mapped onto some kind of L2 transport. My point is that the PPPoE encapsulated packet is further sent over some kind of intermediate transport between the DSLAM (or other broadband aggregator) and the BRAS (...where the PPP is terminated). – rnxrx Dec 3 '16 at 0:05
  • Do you have any external links that corroborate with what you're saying ? – pHeoz Dec 3 '16 at 13:48

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