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| 4.6 |  | PPP and Layer 2 Considerations for Routed and Routing Protocols |
| 4.6.1 |  | Link Control Protocol |

PPP has become the protocol of choice for dial-up, leased lines, and many other point-to-point WAN applications. The reasons for its popularity include multiprotocol support, multilink, address negotiation and compression.

The PPP protocol is broken down into a Layer 2 component called Link Control Protocol (LCP) and a Layer 3 component called the Network Control Protocol (NCP).

The PPP LCP provides a method of establishing, configuring, maintaining, and terminating the point-to-point connection. LCP goes through four distinct phases.

First, link establishment and configuration negotiation occur. Before any network layer datagrams such as IP can be exchanged, LCP first must open the connection and negotiate configuration parameters. This phase is complete when a configuration-acknowledgment frame has been both sent and received.

This is followed by link quality determination. LCP allows an optional link quality determination phase following the link-establishment and configuration-negotiation phase. In this phase, the link is tested to determine whether the link quality is sufficient to bring up network layer protocols. This phase is optional. LCP can delay transmission of network layer protocol information until this phase is complete.

At this point, network layer protocol configuration negotiation occurs. After LCP has finished the link quality determination phase, network layer protocols can be configured separately by the appropriate NCP and can be brought up and taken down at any time. If LCP closes the link, it informs the network layer protocols so that the protocols can take appropriate action.

Finally, link termination occurs. LCP can terminate the link at any time. This usually is done at the request of a user, but can happen because of a physical event such as the loss of carrier or the expiration of an idle-period timer.

Most of the problems that occur with PPP involve the link negotiation. Typically the problems can be classified as follows:

- Authentication
- Incorrect encapsulation at the far end
- Conflicting PPP parameters

The first step in troubleshooting a PPP problem is to check that the appropriate encapsulation is in use with the show interfaces serial command.

Assuming that the correct encapsulation is in use, the next step is to confirm that the LCP negotiations have succeeded by checking the output for LCP Open. Finally, the Layer 3 component can be verified by checking for Open:IPCP or whatever other Layer 3 protocol is expected.

If one of these elements does not appear correct, obtain a detailed debug of the negotiation process using the command debug ppp negotiation. Figure explains in detail each step in a successful PPP link establishment.

One of the most likely problems with bringing up a link is an authentication failure. This will be covered in the next sections.