This chapter shows you how to configure and use the following network security features:
3-1: Modems ” Cisco routers and access servers can use internal or external modems to place and receive calls. Both digital and analog modems can be used. This section presents information on how to configure and manage all types of modems.
3-2: ISDN ” A router can provide dial-in or dial-out service over an ISDN PRI (23 or 30 channels) or an ISDN BRI (two channels).
3-3: Dial-on-Demand Routing (DDR) ” Calls can be placed dynamically from a router to a dialup destination based on specific traffic. DDR provides very flexible and scalable support for multiple dialing interfaces used to reach a destination. Connections can be made and taken down as needed, using dialup resources only when they are necessary.
3-4: Dial Backup ” Dialer interfaces can be used to back up other more permanent or robust interfaces on a router. When the interface goes down, DDR is used to bring up a backup connection automatically. Backup connections can also be made based on the availability of a remote IP route or IP address.
3-5: Routing Over Dialup Networks ” When dialup connections are used between two routers, routing protocol traffic might cause the connection to stay up indefinitely. Snapshot routing performs only periodic routing updates between routers. Routing table entries are frozen until the next periodic update, keeping the dialup connection down unless it is needed for regular traffic.
3-6: Point-to-Point Protocol (PPP) ” PPP can be used on serial interfaces to transport other network-layer protocols over a point-to-point link. PPP offers authentication between endpoints before a connection can be established. Multilink PPP allows multiple physical links to be made, bundling the links into a logical path . Packets are fragmented and distributed over the bundled links. Fragmentation and interleaving can also be used to allow deterministic transport of time-critical data such as voice traffic.
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