Cisco 1003 - 1003 Router Bedienungsanleitung PDF herunterladen (Seite 32)

11-32 Internetwork Design Guide

Security

ISDN includes several features that you can use to increase the security of ISDN connections:

• Callback

• Screening

• Called Party Number Veriﬁcation

• Calling Number Identiﬁcation

Callback

Callback allows a router (typically a remote router) to initiate a circuit-switched WAN link to

another device and request that device to call back. The device, such as a central site router, responds

to the callback request by calling the device that made the initial call. Callback uses the

Point-to-Point Protocol (PPP) and the facilities speciﬁed in RFC 1570. Figure 11-10 shows a typical

negotiation.

Figure 11-10 ISDN Callback

In Figure 11-10, callback is completed in the following sequence of steps:

Step 1 Router A brings up a circuit-switched connection to Router B.

Step 2 Routers A and B negotiate PPP Link Control Protocol (LCP). Router A can request a

callback, or Router B can initiate a callback.

Step 3 Router A authenticates itself to Router B using PPP PAP or CHAP. Router B can

optionally authenticate itself to Router A.

Step 4 Both routers drop the circuit-switched connection.

Step 5 Router B brings up a circuit-switched connection to Router A.

Callback provides centralized billing for synchronous dial-up services. It also allows you to take

advantage of tariff disparities on both a national and international basis. However, because callback

requires a circuit-switched connection to be established before the callback request can be passed, a

small charge (dependent on local tarifﬁng) is always incurred by the router initiating the call that

requests a callback. See “Using ISDN Effectively in Multiprotocol Networks” in the

Internetworking Case Studies publication for a callback conﬁguration example.

Screening

Some central ofﬁce switches support caller ID, which allows the router to verify that an incoming

call comes from an expected source. If you conﬁgure caller ID screening but your central ofﬁce

switch does not support it, the router will reject all calls.

Callback request in LCP

Drop call and callback

Authentication

S4447

Router A

Router B

ISDN

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