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104 Chapter 5: Configuring PPP and Controlling Network Access
PPP LCP
The PPP LCP provides a method of establishing, configuring, maintaining, and terminating the
point-to-point connection. LCP goes through four distinct phases:
1
A link establishment and configuration negotiation occurs. Before any network layer
datagrams (for example, 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.
2
A link-quality determination is made. LCP allows an optional link-quality determination
phase following the link-establishment and configuration-negotiation phase. The link is
tested to determine whether the quality is sufficient to initialize the network layer
protocols. Transmission of network layer protocols can be held until this phase is
complete.
3
The network layer protocol configuration negotiation occurs. Network layer protocols can
be configured separately by the appropriate NCP and can be initialized and taken down at
any time.
4
Link termination then occurs at the request of the user or a predefined inactivity timer, loss
of carrier occurrence, or some other physical event.
Three classes of LCP frames are used to accomplish the work of each of the LCP phases:
·
Link-establishment frames are used to establish and configure a link.
·
Link-termination frames are used to terminate a link.
·
Link-maintenance frames are used to manage and debug a link.
Dedicated and Interactive PPP Sessions
Asynchronous interfaces on an access server can be configured to accept inbound calls from
remote users. There are two modes that can be used in this situation, interactive and dedicated.
In interactive mode, users who dial into the network are able to access the user mode prompt.
The user must enter the command ppp connect to initiate the connection. If access to the router
prompt is unacceptable, dedicated mode should be used. Dedicated mode forces the connection
into a PPP session once the call setup is complete. The command to implement interactive or
dedicated mode for dial-up connections is as follows:
RouterA(config-if)#async mode [dedicated
| interactive]
IP addressing on serial interfaces can be done statically or dynamically. If assigned statically,
the ip address command is used on the interface just as any other interface. To enable dynamic
addressing, the ip unnumbered command is used:
RouterA(config-if)#ip unnumbered interface-type interface-number