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Chapter 8: WAN Protocols and Design
Example 8-1 lists configuration for HDLC, followed by the changed configuration for a
migration to PPP. Assume that Router A and Router B have a serial link attached to their serial
0 ports, respectively.
Changing serial encapsulations in configuration mode is tricky compared to some other
configuration commands in a Cisco router. In Example 8-1, converting back to HDLC (the
default) is done with the
encapsulation hdlc
command, not by using a command such as
no
encapsulation ppp
. Additionally, any other interface subcommands that are pertinent only to
PPP are also removed when the
encapsulation hdlc command is used.
PPP provides several other features in addition to synchronization and framing. The features
fall into two categories: those needed regardless of the Layer 3 protocol sent across the link, and
those particular to each Layer 3 protocol.
The PPP Link Control Protocol (LCP) provides the base features needed regardless of the Layer
3 protocol sent across the link. A series of PPP control protocols, such as IP Control Protocol
(IPCP), provide features for a particular Layer 3 protocol to function well across the link. For
example, IPCP provides for IP address assignment; this feature is used extensively with Internet
dialup connections today.
Only one LCP is needed per link, but multiple control protocols are needed. If a router is
configured for IPX, AppleTalk, and IP on a PPP serial link, the router configured for PPP
encapsulation automatically tries to bring up the appropriate control protocols for each Layer 3
protocol. Table 8-6 summarizes the features of LCP, which performs functions not specific to a
particular Layer 3 protocol.
Example 8-1
Configuration for PPP and HDLC
Router A Router B
Interface serial 0 Interface serial 0
encapsulation ppp encapsulation ppp
. .
. later, changed to... . later, changed to...
. .
interface serial 0 interface serial 0
encapsulation hdlc encapsulation hdlc
Table 8-6
PPP LCP Features
Function
Name of LCP
Feature
Description
Error detection
Link Quality
Monitoring (LQM)
PPP can take down a link based on the percentage of
errors on the link. LQM exchanges statistics about lost
packets versus sent packets in each direction; when
compared to packets and bytes sent, this yields a
percentage of errored traffic. The percentage of loss that
causes a link to be taken down is enabled and defined
by a configuration setting.
ch08.fm Page 524 Monday, March 20, 2000 5:17 PM