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Common ISDN Problems

431

Some ISDN switches require the area code and escape character, even when

the phone numbers are in the same area code. Thus, it is recommended that

dialer-map entries always include the full 11-digit number (North American

Dialing Plan).

Access Lists

Access lists

are commonly used in ISDN connections to prevent certain types

of traffic from triggering a connection. Most frequently, this is done to save
money because ISDN is frequently tariffed on a per-minute, per-B-channel
basis. As of this writing, some providers offered Centrix ISDN and other
options that circumvented this issue, but it is common for Frame Relay and
other technologies to provide the same or greater bandwidth at lower cost.
This is usually true after approximately 40 hours per month or utilization on
the B channels. The xDSL technologies are quickly gaining market share at
an unlimited usage tariff, as well.

To control usage, administrators frequently configure an access list based

on permitted functions only, and all other services are denied. This causes
problems when a new service is added that is not explicitly added to the list.
Troubleshooting any ISDN configuration that worked in the past should
include a thorough review of all access lists, including the dialer lists.

Bottom(config)#

dialer-list 1 protocol ip ?

deny Deny specified protocol

list Add access list to dialer list

permit Permit specified protocol

A dialer list to provide IP, IPX ,and AppleTalk services is shown as follows:

dialer-list 1 protocol ip permit

dialer-list 1 protocol appletalk permit

dialer-list 1 protocol ipx permit

Point-to-Point Protocol

Although the point-to-point protocol (PPP) is recommended for ISDN
connectivity, there are other options available, including the default
HDLC. PPP is recommended in large part to provide security via CHAP,
described previously in this chapter.

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