1.2 Key Characteristics of Scalable Internetworks
1.2.4 Making the network efficient, 1
An efficient network does not waste bandwidth, especially over costly WAN links. To be efficient, routers should prevent unnecessary traffic from traversing the WAN and should minimize the size and frequency of routing updates. The Cisco 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, also called access control lists (ACLs), can be used to prevent traffic that the administrator defines as unnecessary, undesirable, or unauthorized. You may apply one access list on an interface for each protocol, per direction (i.e., in or out). Different filtering policies can be defined for IP, IPX, and AppleTalk. Access lists can also be used to control routing updates, apply route maps, and implement other network "policies" that improve efficiency by curtailing traffic.

Snapshot Routing
Distance-vector routing protocols typically update neighbor routers with their complete routing table at regular intervals. This is done even when nothing has changed in the network's topology. If a remote site relies on a dialup technology, such as ISDN, you can not expect the WAN link to remain active 24 hours a day. In fact, the tolls associated with ISDN make heavy use cost-prohibitive. However, if RIP routers expect updates every 30 seconds by default, the ISDN link would have to be reestablished twice a minute to maintain the routing tables. This hardly seems efficient, especially after employees have gone home for the night. Although it is possible to adjust RIP's timers, the Cisco IOS provides a much better solution to maximize network efficiency in this situation; snapshot routing.

Snapshot routing allows distance-vector routers to exchange their complete tables during an initial connection, but then wait until active periods on the line before again exchanging routing information. The router takes a snapshot of the routing table, which it uses during quiet periods while the dialup link is down. In other words, the routing table is kept frozen so that routes will not be lost because an update has not been 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
The Cisco 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. This increases latency. For this reason, compression proves an efficient measure only on links with extremely limited bandwidth.