CCNA Certification Guide

The process of Legacy DDR works in four steps:

1. Route packets out the interface to be dialed;
2. Determine the subset of the packets that trigger the dialing process;
3. Dial (signal); and
4. Determine when the connection is terminated.

To begin the process of building a Legacy DDR configuration, IP routes are added to the configuration so that packets can be directed out the BRI. This can be accomplished by using the ip route command. You must then define the Legacy DDR logic required to determine when to dial a circuit. This is typically called triggering the dial. Routing packets out the interface to be dialed does not necessarily cause the dial to occur. The Cisco IOS software requires that you define a subset of the packets routed out the interface to actually cause the route to dial. This requires that you define which packets can cause the device to dial. Cisco calls these packets interesting packets. There are two methods that can be used to define interesting packets. In the first method, interesting is defined as all packets of one or more Layer 3 protocols. The second method allows you to define packets as interesting if they are permitted by an access control list (ACL). Access control lists, which are discussed in more detail in Section 8, consider the packets either permitted or denied. With DDR, if the access list permits the packet, it is considered interesting. You can use the dialer-group subcommand to determine what is interesting. This subcommand refers to a dialerlist, which can refer to either an entire protocol suite or an access list.

Before the router can dial, or signal, to set up a call you must configure it with the telephone number of the other router. You can use the dialer string command can be used to tell the router which telephone number to dial. The signalling occurs on the BRI's D channel using Q.931 signalling. When there is only one site to dial, you can configure a single dial string. However, when you have multiple remote sites, the router needs to know each site's telephone number and which telephone number to use when calling each site. Legacy DDR links the number to be dialed to the route that was used to route the packet out the interface. Thus, all that is needed is a mapping between the next-hop addresses and their respective ISDN telephone numbers. This is accomplished by using the dialer map command.

The router keeps an idle timer, which counts the time since the last interesting packet went across the link. It uses this idle timer to determine when to take down the link. If no interesting traffic happens for the number of seconds defined by the idle timer, the router takes the link down. There are two idle timers that must be set: the dialer idle-timeout command can be used to set the idle time, but if the router wants to dial other sites based on receiving interesting traffic for those sites, and all the B channels are in use, another shorter idle timer can be used. The dialer fast-idle command lets you configure a typically lower number than the idle timer so that when other sites need to be dialed, the link that is currently up can be brought down more quickly.

In addition to these commands, the Cisco IOS also expects you to define the type of ISDN switch to which the router is connected. You can use the isdn switch-type command either as a global command or with an interface subcommand to configure this. The types of ISDN switches include:

• basic-net3, which is typically found in Australia, and Europe, including the UK;
• vn3, which is typically found in France;
• ntt, which is typically found in Japan;
• basic-5ess, basic-dms100 and basic-ni1, all of which are typically found in North America.

You might need to configure the Service Profile Identifier (SPID) for one or both B channels, depending on the switch's expectations. When the telephone company's switch has configured SPIDs, it might not allow the BRI line to work unless the router announces the correct SPID values to the switch. SPIDs, when used, provide a basic authentication feature. If your service provider tells you it uses SPIDs, you should configure them, or signalling will not work. The isdn spid command defines the SPIDs for the B channel.