MLPPPoX is negotiated during the LCP session negotiation phase by the presence of the Max-Received-Reconstructed Unit (MRRU) field in the LCP ConfReq. MRRU option is a mandatory field required in MLPPPoX negotiation. It represents the maximum number of octets in the Information field (Data part in Figure: MLPPP encapsulation) of a reassembled packet. The MRRU value negotiated in the LCP phase must be the same on all member links and it can be greater or lesser than the PPP negotiated MRU value of each member link. This means that the reassembled payload of the PPP packet can be greater than the transmission size limit imposed by individual member links within the MLPPPoX bundle. Packets are always be fragmented so that the fragments are within the MRU size of each member link.
Another field that could be optionally present in an MLPPPoX LCP Conf Req is an Endpoint Discriminator (ED). Along with the authentication information, this field can be used to associate the link with the bundle.
The last MLPPPoX negotiated option is the Short Sequence Number Header Format Option which allows the sequence numbers in MLPPPoX encapsulated frames/fragments to be 12-bit long (instead 24-bit long, by default).
After the multilink capability is successfully negotiated by LCP, PPP sessions can be bundled together over MLPPPoX capable links.
The basic operational principles are:
LCP session is negotiated on each physical link with MLPPPoX capabilities between the two nodes.
Based on the ED and the authentication outcome, a bundle is created. A subsequent IPCP negotiation is conveyed over this bundle. User traffic is sent over the bundle.
If a new link tries to join the bundle by sending a new MLPPPoX LCP Conf Request, the LCP session is negotiated, authentication performed and the link is placed under the bundle containing the links with the same ED and authentication outcome.
IPCP and IPv6CP is in the whole process negotiated only once over the bundle. This negotiation occurs at the beginning, when the first link is established and MLPPPoX bundle created. IPCP and IPc6CP messages are transmitted from the 7750 SR LNS without MLPPPoX encapsulation, while they can be received as MLPPPoX encapsulated or non-MLPPPoX encapsulated.