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11-28 Internetwork Design Guide

Performance

This section describes the following techniques for using multiple B channels:

• Bandwidth on Demand

• PPP Multilink

• Dialer Rotary Groups

This section also describes the following techniques for using ISDN bandwidth effectively:

• Compression

• Dial Backup for Leased Lines

Bandwidth on Demand

Bandwidth on demand allows you to use the second B channel of a BRI. When used with a BRI,

bandwidth on demand doubles the bandwidth across the link—from 56/64 kbps to 112/128 kbps.

To conﬁgure an interface to use bandwidth on demand, use the dialer load-threshold interface

conﬁguration command, which speciﬁes a load (a value between 1 and 255) beyond which another

call to the destination is to be initiated. When applied to BRIs, the dialer load-threshold command

brings up the second B channel. In the following partial conﬁguration example, the second B channel

is activated when the ﬁrst B channel is at 50 percent usage.

interface bri 0

ip address 171.68.158.26 255.255.255.248

encapsulation ppp

dialer map ip 171.68.158.25 name RouterA speed 64 14085551111

dialer load-threshold 128

dialer-group 1

ppp authentication chap

Note Bandwidth on demand is a protocol-independent technique and is not speciﬁc to ISDN. The

dialer load-threshold command can only be used with dialer rotary groups. By default, each ISDN

interface is a dialer rotary group.

PPP Multilink

PPP Multilink (MP) is an Internet Engineering Task Force (IETF) standard (RFC 1717) for

aggregating B channels that allows for multivendor interoperability. MP deﬁnes a way of sequencing

and transmitting packets over multiple physical interfaces. To reduce potential latency issues, MP

also deﬁnes a method of fragmenting and reassembling large packets that can be used for protocols

that do not tolerate out-of-order packets, such as AppleTalk and IPX.

It is important to note that MP does not deﬁne how or why connections should be initiated or torn

down. The design of this mechanism has been left to the vendors. Cisco’s implementation allows the

user to deﬁne a load factor (the percentage of bandwidth being used on a B channel) at which point

a second or subsequent B-channel call should be initiated. The load factor can be deﬁned for

incoming data only, outgoing data only, or for both. This allows MP to be used effectively in many

different environments, such as collecting information from the World Wide Web (mostly incoming

trafﬁc) and sending ﬁles to colleagues (mostly outgoing trafﬁc).

Figure 11-8 illustrates a basic MP session in which MP has already brought a second B channel into

operation. Incoming packets A and B are both fragmented into smaller packets, given a sequence

number (A1, A2, B1, and B2), and shared over the two B channels. (All packets greater than 30 bytes

1 2 ... 23 24 25 26 27 28 29 30 31 32 33 34

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