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PPPoE Discovery Stage
1. The PPPoE client broadcasts a PADI packet that contains information about the service type
it requests.
2. PPPoE IA intercepts PPPoE discovery frames from the client and inserts a unique line
identifier (circuit-id/remote-id) using the PPPoE Vendor-Specific tag (0x0105) to PADI
(PPPoE Active Discovery Initiation) packets. The PPPoE IA forwards these packets to the
PPPoE server after the insertion.
3. After receiving a PADI packet that it can serve, a PPPoE server replies with a PADO packet.
The destination address of the PADO packet is the unicast packet of the host that sent the
PADI.
4. Depending on the network topology, since the PADI was broadcast, the PPPoE client may
receive PADO packets sent by multiple PPPoE servers. Among these PPPoE servers, the
PPPoE client selects the one whose PADO packet arrived the earliest and unicast’s a PADR
packet to the PPPoE server.
5. Depending on the network topology, since the PADI was broadcast, the PPPoE client may
receive PADO packets sent by multiple PPPoE servers. Among these PPPoE servers, the
PPPoE client selects the one whose PADO packet arrived the earliest and unicasts a PADR
packet to the PPPoE server.
6. PPPoEIA intercepts PPPoE discovery frames from the client and inserts a unique line
identifier (circuit-id/remote-id) using the PPPoE Vendor-Specific tag (0x0105) to PADR
(PPPoE Active Discovery Request) packets. The PPPoE IA forwards these packets to the
PPPoE server after the insertion.
7. The PPPoE server generates a unique session ID for the session and sends the session ID to
the PPPoE client through a PADS packet. If no error occurs, the session will thus be
established and PPPoE moves on to the Session stage.
PPPoE Intermediate Agent Tag Format
If the PPPoE Intermediate Agent is enabled, the Switch adds a vendor-specific tag to PADI
(PPPoE Active Discovery Initialization) and PADR (PPPoE Active Discovery Request) packets
from PPPoE clients. This tag is defined in RFC 2516 and has the following format for this feature.
Table 1 PPPoE Intermediate Agent Vendor-specific Tag Format
Tag_Type
(0x0105)
Tag_Len Value #1 #2
The Tag_Type is 0x0105 for vendor-specific tags, as defined in RFC 2516. The Tag_Len
indicates the length of Value, #1 and #2. The Value is the 32-bit number
0x00000DE9
, which
stands for the “ADSL Forum” IANA entry. #1 and #2 are PPPoE intermediate agent sub-options,
which contain additional information about the PPPoE client.
Sub-Option Format
There are two types of sub-option: “Agent Circuit ID Sub-option” and “Agent Remote ID Sub-
option”. They have the following formats.
Table 2 PPPoE IA Circuit ID Sub-option Format: User-defined String
SubOpt Length Value
N
(1 byte)
0x01
(1 byte)
String
(64 bytes)