1.2 Overview of Scalable Internetworks
1.2.5 Making the network efficient (cont.)

Additional Cisco IOS features that can help optimize bandwidth use are as follows:

  • Dial-on-demand routing (DDR)
  • Switched access
  • Route summarization
  • Incremental updates

Dial-on-demand routing (DDR)
Connections for infrequent traffic flow can be accomplished using DDR. In DDR, active links are created only after interesting traffic is detected by the router. This "only as required" service replaces dedicated circuits that are charged for even when that link is idle.

Switched access
Switched access provides the capability to enable a WAN link on an as-needed basis via automated router controls. One model for a reliable backbone consists of dual, dedicated links and one switched link for idle hot backup. Under normal operational conditions, you can load balance over the dual links, but the switched link is not operational until one of the dedicated links fails.

Traditionally, WAN connections over the Public Switched Telephone Network (PSTN) have used dedicated lines. Packet-switched networks, such as X.25 and Frame Relay, offer the advantage of providing global connectivity through a large number of service providers with established circuits to most major cities. Packet-switched networks enable nodes to share the bandwidth by sending data in packets.

You can reduce the number of router processing cycles by reducing the overall number of routing entries in a router's routing table. This can be done using the following Cisco IOS features:

Route summarization
The number of entries in a routing table can be reduced by using route aggregation or, as it is more commonly known, route summarization. Summarization of routes occurs at major network boundaries for most routing protocols by simply finding the common bits or bytes of addresses that a router advertises. Instead of advertising all the routes, the router only advertises the common bits or bytes (or summary) of the routes. Some IP routing protocols, such as OSPF and Enhanced IGRP, allow manual summarization on arbitrary boundaries within the major network. Careful planning and address allocation is required for route summarization to be most effective.

Incremental updates
Protocols such as Enhanced IGRP and OSPF make more efficient use of bandwidth than distance vector protocols by sending only topology changes rather than the entire routing table contents at fixed intervals.