The route table acquires information in two ways. The information may be entered manually, by means of
a static route entry, or automatically by one of several systems of automatic information discovery and
sharing known as dynamic routing protocols. The bulk of this book concerns dynamic IP routing
protocols, but this discussion of static route configuration will prepare you to understand the subsequent
chapters.
More to the point, static routing is preferred over dynamic routing in certain circumstances. As with any
process, the more automatic it is, the less control we have over it. Although dynamic (automatic) routing
requires much less human intervention, static routing allows very precise control over the routing
behavior of an internetwork. The price to be paid for this precision is the necessity of manual
reconfiguration any time the topology of the network changes.
NOTE
Discontiguous networks
NOTE
Variable subnetting
Case Study: Simple Static Routes
Figure 3.3 shows an internetwork with four routers and six networks. Notice that the subnets of network
10.0.0.0 are discontiguous—there is a different major network subnet (192.168.1.192, in the Tigger-to-
Piglet link) separating 10.1.0.0 from the other 10.0.0.0 subnets. The subnets of 10.0.0.0 are also variably
subnetted—the subnet masks are not consistent throughout the internetwork. Finally, the subnet address
of Pooh's Ethernet link is an all-zero subnet. Later chapters demonstrate that an addressing scheme with
these characteristics causes problems for simpler, classful routing protocols such as RIP and IGRP; static
routes work fine here.
Figure 3.3. Routing protocols such as RIP and IGRP cannot easily route this discontiguous, variably
subnetted internetwork, but static routing will work.
The routing commands themselves are easily read if the reader remembers that each command describes a
route table entry. The command is ip route, followed by the address to be entered into the table, a mask
for determining the network portion of the address, and the address of the directly connected interface of
the next-hop router.
An alternative configuration command for static routes specifies the interface out of which a network is
reached instead of the address of the next-hop router. For example, the route entries for Tigger could be
as follows:
Tigger(config)# ip route 192.168.1.0 255.255.255.224 S0
Tigger(config)# ip route 10.1.0.0 255.255.0.0 E0
Tigger(config)# ip route 10.4.7.0 255.255.255.0 S1
Figure 3.4 compares the route table resulting from this configuration with the route table resulting from
entries pointing to a next-hop router. Notice that a certain inaccuracy is introduced; all networks specified
with a static route referring to an exit interface are entered into the table as if they are directly connected
to that interface. The implications for route redistribution are discussed in Chapter 11.
Figure 3.4. The top route table is the result of static route entries pointing to the next-hop router. The
bottom route table is the result of static routes that point to the interface a packet must exit to reach the
destination network.[4]