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.