PPP is a versatile protocol that provides a designer with many options when deploying it in the
network. Through the use of interactive PPP, you can provide flexibility to dial-in users using
asynchronous interfaces. If you are looking for a more secure dial-in environment, you can configure
an asynchronous interface to run in dedicated PPP mode. Another option for the security
conscious is to utilize PPP callback for enhanced security.
Addressing can be configured by using static IP addresses, but for greater flexibility we suggest
using DHCP or address pools to automatically assign IP addressing. User authentication
can be accomplished by using Password Authentication Protocol (PAP) or its more secure
cousin, Challenge Handshake Authentication Protocol (CHAP).
Compression and multilink are two PPP options that a network designer can use to increase
traffic flow on a connection. They can be used separately or together for even greater throughput.
When trouble occurs with an asynchronous connection, Cisco IOS offers a variety of troubleshooting
commands to assist the administrator in narrowing down the problem. These commands
can be used to show the PPP negotiation process, PPP authentication process, or each
PPP packet as it traverses an interface.
Know how to set up PPP on an interface and know what LCP and NCP are used for. To
configure PPP encapsulation on an interface, use the encapsulation ppp command. LCP is
used for PPP link control, including circuit testing and authentication. NCP is used to negotiate
which upper layer protocols will run over a connection and negotiates their addressing.
Understand how to set up an interface to allow multiple protocols and how to restrict the
method of access to a single protocol. An interface can be set up with the command async
mode interactive to enable a user to choose between SLIP and PPP encapsulation. The
administrator can restrict the user from changing the encapsulation by using the async mode
dedicated command. By using the autoselect command to sense the desired protocol, the
router can automatically configure the interface.
Know the three methods to give an interface an IP address. You can use static IP addressing,
which requires more administrator overhead; IP unnumbered, which has troubleshooting problems;
or dynamic IP address allocation using DHCP or IP address pools.
Be able to give a general overview of how DHCP works. The DHCP client uses a broadcast
packet to communicate with a DHCP server. A negotiation process determines an IP address
lease. After half of the lease time has expired, the DHCP client will attempt to renew the lease.
Understand the differences between the two PPP authentication protocols and when to use them.
You should understand how the PAP and CHAP protocols work and why CHAP is the better protocol
to use. CHAP never sends the username and password over the link, whereas PAP sends both
in cleartext. Security can be compromised with PAP, but sometimes legacy systems require its use.
Know the commands to use when configuring compression and Multilink PPP (MPPP). Use the
compress [predictor | stac | mppc [ignore-pfc]] command to configure compression on
a link, making sure to use the same method on each end of a connection. You can bond multiple
channels into a single connection for greater speed by using the ppp multilink command. Compression
and multilink can be used together or separately to enhance connection throughput.
Know the PPP troubleshooting commands and how to spot a problem. You can use the
debug ppp authentication, debug ppp negotiation, and debug ppp packet commands to
determine the cause of a remote access problem.