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11-30 Internetwork Design Guide

Performance

Compression

The Point-to-Point (PPP) Compression Control Protocol (CCP) is an Internet Engineering Task

Force (IETF) draft RFC that deﬁnes a method for negotiating data compression over PPP links.

These links can be either leased lines or circuit-switched WAN links, including ISDN. Compression

increases throughput and shortens ﬁle transfer times.

Use the compress interface conﬁguration command at both ends of the link to enable compression.

Use the stac keyword to enable the Stacker (LZS) compression algorithm or the predictor keyword

to enable the RAND algorithm (a predictor algorithm). The Stacker algorithm is appropriate for

LAPB and PPP encapsulation, and the RAND algorithm is appropriate for HDLC and PPP

encapsulation. The Stacker algorithm is preferred for PPP encapsulation.

Dial Backup for Leased Lines

Dial backup protects against wide-area network (WAN) downtime by allowing a dedicated serial

connection to be backed up by a circuit-switched connection. To conﬁgure dial backup, associate an

interface (in this case, an ISDN interface) as a backup to a primary serial interface.

Once conﬁgured, the dial backup interface remains inactive until one of the following conditions is

met:

• The primary line goes down—When the Carrier Detect signal from the primary line device is lost

or when the line protocol goes down, the backup line is activated, preserving the connection

between the two sites.

• The transmitted trafﬁc load on the primary line exceeds a deﬁned limit—The trafﬁc load is

monitored and a 5-minute moving average is computed. If the average exceeds the user-deﬁned

value for the line, the backup line is activated. Depending on how the backup line is conﬁgured,

some or all of the trafﬁc ﬂows onto it.

The following interface conﬁguration commands, applied to the primary interface, establish a dial

backup:

• The backup interface interface conﬁguration command speciﬁes the interface that is to act as

the backup.

• The backup load command speciﬁes the trafﬁc threshold at which the backup interface is to be

activated and deactivated.

• The backup delay command speciﬁes the amount of time that is to elapse before the backup

interface is activated or deactivated after a transition on the primary interface.

Note An interface can be conﬁgured as the backup for one interface only—it cannot be conﬁgured

as the backup for multiple interfaces.

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