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| 1.2 |  | Key Characteristics of Scalable Internetworks |
| 1.2.4 |  | Making the network efficient |

An efficient network should not waste bandwidth, especially over costly WAN links. To be efficient, routers should prevent unnecessary traffic from traversing the WAN and minimize the size and frequency of routing updates. The IOS includes several features designed to optimize a WAN connection:

- Access lists
- Snapshot routing
- Compression over WANs

The following sections describe each of these features.

Access Lists
Access lists, Figure , also called Access Control Lists (ACLs), can be used to do all of the following:

- Prevent traffic that the administrator defines as unnecessary, undesirable, or unauthorized
- Control routing updates
- Apply route maps
- Implement other network policies that improve efficiency by curtailing traffic

One access list may be applied on an interface for each protocol, per direction, in or out. Different filtering policies can be defined for IP, IPX, and AppleTalk.

Snapshot Routing
Distance vector routing protocols typically update neighbor routers with their complete routing table at regular intervals. These timed updates occur even when there have been no changes in the network topology since the last update. If a remote site relies on a dialup technology, such as ISDN, it would be cost prohibitive to maintain the WAN link in an active state 24 hours a day. RIP routers expect updates every 30 seconds by default. This would cause the ISDN link to reestablish twice a minute to maintain the routing tables. It is possible to adjust the RIP timers, but snapshot routing provides a better solution to maximize network efficiency.

Snapshot routing allows routers using distance vector protocols to exchange their complete tables during an initial connection. Snapshot routing then waits until the next active period on the line before again exchanging routing information. The router takes a snapshot of the routing table. The router then uses this picture for routing table entries while the dialup link is down. The result is that the routing table is kept unchanged so that routes will not be lost because a routing update was not received. When the link is re-established, usually because the router has identified interesting traffic that needs to be routed over the WAN, the router again updates its neighbors.

Compression over WANs
The IOS supports several compression techniques that can maximize bandwidth by reducing the number of bits in all or part of a frame. Compression is accomplished through mathematical formulas or compression algorithms. Unfortunately, routers must dedicate a significant amount of processor time to compress and decompress traffic, increasing latency. Therefore, compression tends to be an efficient measure only on links with extremely limited bandwidth.

The IOS also supports the following bandwidth optimization features:

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

Dial-on-Demand Routing
Dedicated WAN circuits, even Frame Relay, may be cost prohibitive for every remote site. DDR offers an efficient, economic alternative for sites that require only occasional WAN connectivity. A DDR WAN link is not a dedicated link that is always on. The DDR link will activate and establish the WAN connection when interesting traffic is sent over the WAN link by way of the WAN router. For example, in Figure , a router configured for DDR will listen for interesting traffic and wait to establish the WAN link. The administrator defines what is interesting traffic. When the router receives traffic that meets the criteria, the link is activated. DDR is most commonly used with ISDN circuits.

in Figure [2], a router configured for DDR will listen for interesting traffic and wait to establish the WAN link. Again, the administrator will define what is interesting traffic. When the router receives traffic that meets the criteria, the link is activated. DDR is most commonly used with ISDN circuits.

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. Route summarization is presented in Module 2.

Incremental updates
Routing protocols such as OSPF, IS-IS, 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 that transmit their complete routing table at fixed intervals, whether a change has occurred or not.