Cisco 1003 - 1003 Router Bedienungsanleitung PDF herunterladen (Seite 23)

Designing ISDN Internetworks 11-23

Tariff Management

When the client router transitions from the quiet period to the active period, the line might be down

or busy. If this happens, the router would have to wait through another entire quiet period before it

could update its routing table, which might severely affect connectivity if the quiet period is very

long. To avoid having to wait through the quiet period, snapshot routing supports a retry period. If

the line is not available when the quiet period ends, the router waits for the amount of time speciﬁed

by the retry period and then transitions to an active period.

The retry period is also useful in dial-up environments in which there are more remote sites than

interface lines. For example, the central site might have one PRI (with 23 B channels available) but

might dial more than 23 remote sites. In this situation, there are more dialer map commands than

available lines. The router tries the dialer map commands in order and uses the retry time for the lines

that it cannot immediately access.

Enabling Snapshot Routing

Snapshot routing is enabled through interface conﬁguration commands. The central router is

conﬁgured for snapshot routing by applying the snapshot server interface conﬁguration command

to its ISDN interfaces. The snapshot server command speciﬁes the length of the active period and

whether the router is allowed to dial remote sites to exchange routing updates in the absence of

regular trafﬁc.

The remote routers are conﬁgured for snapshot routing by applying the snapshot client command to

each ISDN interface. The snapshot client interface conﬁguration command speciﬁes the length of

the active period (which must match the length speciﬁed on the central router), the length of the quiet

period, whether the router can dial the central router to exchange routing updates in the absence of

regular trafﬁc, and whether connections that are established to exchange user data can be used to

exchange routing updates.

For a snapshot routing conﬁguration example, see the chapter “Using ISDN Effectively in

Multiprotocol Networks” in the Internetworking Case Studies publication.

Using Snapshot Routing with Enhanced IGRP

Because it sends out “hello” packets every 5 seconds to maintain its routing tables, Enhanced IGRP

routing protocol updates need to be controlled over ISDN links. Because snapshot routing does not

control Enhanced IGRP packets, another technique must be used. One way is to use access lists, as

described in the “IP Enhanced IGRP Packets” section later in this chapter.

Another way is to use RIP (which snapshot routing can control) across ISDN links. For the RIP

approach to work, RIP information must be redistributed into the Enhanced IGRP networks and vice

versa. Figure 11-7 shows a simple network in which IP Enhanced IGRP is used on both LANs with

RIP running between the two LANs.

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Cisco 1003 - 1003 Router Bedienungsanleitung Seite 23