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Cisco ISDN Configuration

Summary This document contains an example Cisco ISDN Configuration.

Keywords Cisco ISDN Configuration

Description Cisco ISDN Configuration

Configuration Example

version 12.0
service config
no service pad
service tcp-keepalives-out
! Generates TCP keepalives to bring the
! BRI interface up on system startup and
! after disconnection.
service timestamps debug uptime
service timestamps log datetime
! This enables logging of events, such as
! access-list blocks and ISDN line changes.
service password-encryption
! Encrypts passwords so that they don't
! appear as plaintext in the configuration.
hostname training
enable secret 5 _password_
ip subnet-zero
ip classless
ip name-server
ip name-server
isdn switch-type basic-ni
interface Ethernet0
 ip address x.x.x.x
 no ip directed-broadcast
interface BRI0
 no ip address
! newr IOS's may use: ip address negotiated
no ip directed-broadcast
encapsulation ppp
dialer rotary-group 0
isdn spid1 304yyyyyyy0101
isdn spid2 304zzzzzzz0101
isdn timeout-signaling
! Flushes the ISDN D channel if no switch
! response is received within 1 second. This
! is only available on newer IOS's.
no fair-queue
interface Dialer0
no ip address
no ip directed-broadcast
! Prevents any incoming traffic to a
! directed broadcast (all 1's in the
! host bits), keeping any networks behind
! this router from being used as a denial of
! service amplification network.
encapsulation ppp
dialer in-band
dialer string 3262729
dialer hold-queue 70
dialer load-threshold 1 either
dialer-group 1
no fair-queue
ppp authentication pap callin
ppp pap sent-username matt1 password 7 11041811030B5A
ppp multilink
ip route Dialer0
dialer-list 1 protocol ip permit
! Permits all IP traffic to trigger dialout
line con 0
 exec-timeout 0 0
 transport input none
 stopbits 1
line vty 0 4
 password _password_

Changing the username and password
Changing the username and password that a Cisco ISDN router uses to dial-up to the Internet is fairly easy thing. However, you do need to be in priveledged exe mode and you need to be able to see the running configuration. To change the username and password in this example, be certain that the username and password are in the Dialer0 interface with the configuration line of:

ppp pap sent-username matt1 password 7 11041811030B5A

To change that line, you need to get into enable and remove it. Then, you can add the appropriate line. This would look like:

Enter password:
router#conf t
router(config)#interface Dialer0
router(config-if)#no ppp pap sent-username matt1 password 7 11041811030B5A
router(config-if)#ppp pap sent-username matt2 password password

Once you have done that, press [CTRL-z] to exit the configuration. Then, save the configuration by running the copy running-config startup-config command. Finally, either have the router disconnect and reconnect or power cycle it.

Changing the dial number
Like the above example, you need to remove the old configuration line and add the new one before the change will work. To change the dial number, do the following:

Enter password:
router#conf t
router(config)#interface Dialer0
router(config-if)#no dialer string 3049991243
router(config-if)#dialer string 3048889786

That will remove the old configuration line and put in its place a new dialer number in the Dialer0 interface.

Changing the telnet and enable passwords
Changing the telnet and enable passwords for Cisco routers is similar to the above examples. However, there are several different ways that this can be accomplished. Again, like the previous examples, you need to see the running configuration to see how it is currently configured. In this example, change the telnet password by doing the following:

Enter password:
router#conf t
router(config)#line vty 0 4
router(config-line)#no password password
router(config-line)#password password

The line vty 0 4 command that was issued put the router into the line that controls telnet logins. Then, the old password was removed. Although this is not necessary, it has been noted that some routers are picky about that step. Finally, the new password was added to the vty line.

For the enable password in this configuration, there are actually two lines that affect this:

enable secret 5 _password_
enable password _password_

The enable secret line stores the password in the router configuration in encrypted format, so even if you can view the running configuration, you will be unable to see the password. The enable password line stores the password in plaintext. If an enable secret password and an enable password are both configured, then the Cisco router uses the enable secret line by default.

To change the enable password in this example and do a bit of clean up, do the following:

Enter password:
router#conf t
router(config)#no enable password
router(config)#no enable secret
router(config)#enable secret password

Troubleshooting connectivity problems
The table below lists many commands that can be used to troubleshoot ISDN connectivity problems on a Cisco router. Reading the output of the commands is not difficult, but remembering which commands to use can be, so this list below will help you out.

Command Description
show interface BRI0 Shows the statistics, such as packets sent and received and the number of CRC's on the BRI interface. This is useful in seeing the amount of errors that the BRI interface is receiving. It is best used in conjunction with the clear counters BRI0 command so that the information listed is current.
show dialer Lists the general diagnostic information about an interface configured for DDR, such as the number of times the dialer string has been successfully reached, and the idle timer and fast idle timer values for each B channel. Current call-specific information is also provided, such as the length of the call, and the number and name of the device to which the interface is currently connected.
show isdn active Use to show if a call is in progress and which number is being dialed.
show isdn status Shows the statistics of the ISDN connection.
show ip route Displays the routes known to the router, including static and dynamically learned routes.
debug isdn q921 Verfies that you have a connection to the ISDN switch.
debug isdn q931 Displays call setup and teardown messages.
debug dialer Shows information such as what number the interface is dialing.
shutdown and
no shutdown
These commands can be used to disconnect any call in progress and reset an interface.
clear interface BRI0    This command causes the BRI interface to reset itself, re-establish a connection to the switch and then dial-up to the number that it is configured for.