The Link Control Protocol (LCP) is used to establish the connection through an exchange of Configure packets. This exchange is complete, and the LCP Opened state entered, once a Configure-Ack packet (described later) has been both sent and received.
All Configuration Options are assumed to be at default values unless altered by the configuration exchange. See the chapter on LCP Configuration Options for further discussion.
It is important to note that only Configuration Options which are independent of particular network-layer protocols are configured by LCP. Configuration of individual network-layer protocols is handled by separate Network Control Protocols (NCPs) during the Network-Layer Protocol phase.
Any non-LCP packets received during this phase MUST be silently discarded.
The receipt of the LCP Configure-Request causes a return to the Link Establishment phase from the Network-Layer Protocol phase or Authentication phase.