How PPP packet travels from CPE to BRAS edge router ?

[Ravi VK]

Hi ,

CPE <--------------> DSLAM -------------> BRAS

Once PPPoE/PPP LCP and NCP negotiations are done , CPE and BRAS know each others MAC addresses. When a data packet encapsulated in PPP/PPPoE is sent from CPE , it will have Source MAC (say SrcMAC1) and destination MAC of BRAS (say DstMAC1) . Packet is sent from CPE to DSLAM . Will DSLAM put its source MAC here ? As per my knowledge DSLAM will not add its source mac .

Please clarify , how packet is forwarded from CPE to BRAS and vice a versa.

[Amit Goyal]

Hi Ravi,

Let me correct the topology.

CPE-------ATM---------DSLAM-----(ATM or Ethernet)-----------(BRAS)

>Most of the time CPE to DSLAM is an ATM connectivty. CPE runs PPPOA (PPP over ATM).

>When the data is received by the Digital Subscriber Line Access Multiplexer (DSLAM). The DSLAM aggregates the digital data streams coming from a number of subscribers onto a single high-capacity uplink (ATM or

Gigabit Ethernet backhaul) to the Internet Service Provider. At the ISP the aggregated data from multiple subscribers is processed by the Broadband Remote Access Server (B-RAS) which authenticates the subscriber’s credentials,

validates the users access policies and routes the data to its respective destinations on the Internet.

>For sure end goal is to have a PPP session between CPE and BRAS but this PPP rides over multiple encapsulation. ATM PVC and Ethernet MAC address are among them.

Hope it helps

-Amit

[Ravi VK]

Hi Amit ,

Thanks for the explanation .

What if we have ethernet everywhere i.e 

CPE <---Ethernet------> DSLAM <------Ethernet-----> BRAS.

My initial question is based on above topology .

Correct me if my understanding is wrong.

[Amit Goyal]

Hi Ravi,

I will prepare a LAB and let you know about it.

-Amit

[Masoud Pourshabanian]

Hello,

Amit explanation was nice, I am just adding more info.

Between CEP and DSLAM there is a DSL connection.

CPE <---Ethernet------>DSL modem --telephone line--- DSLAM <------Ethernet-----> BRAS.

1- CPE encapsulates an IP packet in PPP header, PPP in PPPOE header, PPPOE in ethernet then sends it to DSL modem.

2- DSL modem add AAL5 header, chops it into cells, adds DSL header and sends cells(53 bites) to DSLAM over telephone line(DSL connection).

3- DSLAM receives the cells, removes the ATM headers, puts cells together, and sends the original ethernet frame to BRAS. BRAS replies and the same procedure repeats.

Please check the picture below to get the idea.

http://flylib.com/books/3/475/1/html/2/images/0131777203/graphics/09fig01.gif

Hope it helps,

Masoud

[Ravi VK]

Thanks for the detailed explanation Masoud.