6.3 Time Based Access Lists
6.3.2 Application: Time Based ISDN
Time-based Access Control Lists (ACL) are used to implement time-based Dial-on-Demand Routing (DDR). These ACLs help control dialing when they are used to define what traffic is interesting enough to cause or maintain a dial-up connection. This mechanism can be useful to control undesired dialing that could generate high charges by a telephone company. The definition of interesting traffic is part of the DDR feature present in Cisco IOS® Software. This feature is independent of the actual dial-up technology being used (ISDN or plain old telephone service (POTS)).

The implementation of time-based access lists requires two tasks. These are to define a time range and then reference a time range. To define a time range, use the commands shown in Figure beginning in global configuration mode. Repeat these tasks if you have multiple items you want in effect at different times. For example, repeat the steps to include multiple permit or deny statements in an access list in effect at different times.

In order for a time range to be applied, you must reference it by name in a feature that can implement time ranges. To reference the time range, create either an IP extended access list or named access list (see Figure ). After creating an access list, you must then reference it to make it work.