HCC Cisco Academy: Sample ISDN BRI Configuration

ISDN configuration may be made on a plain Serial router port, but usually is done on a Basic Rate Interface (BRI) or a Primary Rate Interface (PRI).  (The difference is a single command, dialer in-band.  Use this command on a Serial interface to let the router know the dial information is not using the out-of-band D-channel available in a BRI or PRI interface.)

The Correct use of ISDN usually requires PPP to be configured as well, including authentication.  Finally, DDR (Dial on Demand Routing) is usually configured.  DDR can be set up in two ways, the method known as Leagacy DDR is discussed below (the other is called DDR dialer profiles).

The Academy Semester 4 material on ISDN covers the topics but fails to provide a complete example configuration.  Shown below is the configuration commands related to the setup of a BRI interface on RouterA to RouterB, using a PPP CHAP password of boardwalk (don't use this password yourself) and a phone number of 555-1234.  The IP addresses used for this link are (RouterA) and (RouterB), with a subnet mask of  The network we're trying to reach on the far side of RouterB's interface is

Some comments (lines starting with a "!") have been added to explain commands not stressed in the course.

Note that the dial string is not necessarily the same as the SPID (or SPIDs).  If the interface can be used to dial a single location only, you can use the dialer string 5551234 command.  However if the interface can be used for more than one destination you need to map the destination address to a phone number, and a slightly more complex command (used below even though for now only a single destination, RouterB, is configured) is used. 

In this example we assume the switch type is 5ess and the SPIDS are 12345 and 54321.

The interesting traffic that will bring up the link will be any IP traffic.  While a simpler form of the dialer-list could be used this this case, we will define and use an access-list as an example.

The link will be brought down if no interesting traffic appears for idle-timeout seconds.  However, if the link is used for more than one dial out destination, it is possible that there is interesting traffic for the second destination while the first link is still up and waiting for the idle-timeout period to expire.  In this situation a second timer can be used to terminate the established link more quickly, the fast-idle timeout value.  (This has been configured below, even though the BRI interface is used for a single outgoing destination only.)

RouterA(config)# ! Set the PPP CHAP password for the link to RouterB:
RouterA(config)# username RouterB password boardwalk
RouterA(config)# ! Set up ACL for "interesting" traffic:
RouterA(config)# access-list 101 permit ip any
RouterA(config)# dialer-list 2 protocol ip list 101
RouterA(config)# ! Set up static route to far end of PPP link:
RouterA(config)# ip route
RouterA(config)# interface bri 0/0
RouterA(config-if)# ip address
RouterA(config-if)# encapsulation ppp
RouterA(config-if)# ppp authentication chap
RouterA(config-if)# ! Setup load sharing (to use full ISDN bandwith)
RouterA(config-if)# ! whenever the utilization of the currently used links
RouterA(config-if)# ! is more that 25 percent:
RouterA(config-if)# ppp multilink
RouterA(config-if)# dialer load-threshold 25 either
RouterA(config-if)# ! define switch type (may be done globally) and SPIDS:
RouterA(config-if)# isdn switch-type basic-5ess
RouterA(config-if)# isdn spid1 12345
RouterA(config-if)# isdn spid2 54321
RouterA(config-if)# ! Define the dial map(s) and apply the dialer-list:
RouterA(config-if)# dialer map ip broadcast name RouterB 5551234
RouterA(config-if)# dialer-group 2
RouterA(config-if)# ! Determine when the link should be dropped:
RouterA(config-if)# dialer idle-timeout 300
RouterA(config-if)# dialer fast-idle 30

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