Overview of Sequencing NCP Packets for POS Interfaces with PPP Encapsulation

For packet over SONET (POS) physical interfaces, you can configure PPP as the encapsulation method on the interface to enable a PPP dialogue to be initiated between the source and the destination points. After the completion of the link-establishment and authentication phases in a PPP session between two routers are completed, the network phase begins and the PPP connection is fully established. At this point, any higher level protocols (for example, IP protocols) can initialize and perform their own negotiations and authentication.

After a link has been established and optional facilities have been negotiated as needed by the LCP, PPP must send Network Control Protocol (NCP) packets to choose and configure one or more network-layer protocols, such as IP, IPX, or AppleTalk. Once each of the chosen network-layer protocols has been configured, datagrams from each network-layer protocol can be sent over the link. NCP initializes the PPP protocol stack to handle multiple network layer protocols, such as IPv4, IPv6, and Connectionless Network Protocol (CLNP).

To enable the PPP session to be established only when the NCP packets arrive in an ordered format on POS interfaces that are configured with PPP encapsulation, you can enter the ppp ncp-Ordering-Required command in Global Configuration mode. Otherwise, the POS interfaces with PPP encapsulation configured remain in the down state and are disabled unless the NCP packets arrive in a sequenced way. If you want the PPP session to be established and the POS interfaces to move to the up state even when the NCP packets arrive in an out-of-order format, you must enter the no ppp ncp-Ordering-Required command for the POS interfaces to be activated.