1.2 Key Characteristics of Scalable Internetworks
1.2.5 Making the network efficient, 2
The Cisco IOS also supports the following bandwidth optimization features:
  • Dial-on-demand routing (DDR)
  • Switched access
  • Route summarization
  • Incremental updates

Dial-on-Demand Routing
An organization cannot always afford dedicated WAN circuits, or even Frame Relay, for every remote site. At sites that require only occasional WAN connectivity, dial-on-demand routing (DDR) offers an efficient, economical alternative. As shown in the figure, a router configured for DDR will listen for interesting traffic and wait to build the WAN link. When the router receives interesting traffic (as defined by the administrator), it places a call to activate the link, which is commonly ISDN.

Route Summarization
The number of entries in a routing table can be reduced if the router uses one network address and mask to represent multiple networks or subnetworks. This technique is called route aggregation, or route summarization. Some routing protocols automatically summarize subnet routes based on the major network number. Other routing protocols, such as OSPF and EIGRP, allow manual summarization. You will learn more about route summarization in the next chapter.

Incremental Updates
Some routing protocols, such as OSPF and EIGRP, send routing updates that contain information only about routes that have changed. These incremental routing updates use the bandwidth more efficiently than simple distance-vector protocols, which transmit their complete routing table at fixed intervals, regardless of whether a change has occurred.