A-9
Cisco 3900 Series, Cisco 2900 Series, and Cisco 1900 Series Integrated Services Routers Generation 2 Software Configuration Guide
Appendix A Cisco IOS CLI for Initial Configuration
Using the Cisco IOS CLI to Perform Initial Configuration
Default Routes
A router might not be able to determine the routes to all other networks. To provide complete routing
capability, the common practice is to use some routers as smart routers and give the remaining routers
default routes to the smart router. (Smart routers have routing table information for the entire
internetwork.) These default routes can be passed along dynamically, or can be configured into the
individual routers.
Most dynamic interior routing protocols include a mechanism for causing a smart router to generate
dynamic default information that is then passed along to other routers.
Default Network
If a router has an interface that is directly connected to the specified default network, the dynamic
routing protocols running on the router will generate or source a default route. In the case of RIP, the
router will advertise the pseudo network 0.0.0.0. In the case of IGRP, the network itself is advertised and
flagged as an exterior route.
A router that is generating the default for a network also may need a default of its own. One way a router
can generate its own default is to specify a static route to the network 0.0.0.0 through the appropriate
device.
Gateway of Last Resort
When default information is being passed along through a dynamic routing protocol, no further
configuration is required. The system periodically scans its routing table to choose the optimal default
network as its default route. In the case of RIP, there is only one choice, network 0.0.0.0. In the case of
IGRP, there might be several networks that can be candidates for the system default. The Cisco IOS
software uses both administrative distance and metric information to determine the default route
(gateway of last resort). The selected default route appears in the gateway of last resort display of the
show ip route EXEC command.
If dynamic default information is not being passed to the software, candidates for the default route are
specified with the ip default-network global configuration command. In this usage, the ip
default-network command takes an unconnected network as an argument. If this network appears in the
routing table from any source (dynamic or static), it is flagged as a candidate default route and is a
possible choice as the default route.
If the router has no interface on the default network, but does have a route to it, it considers this network
as a candidate default path. The route candidates are examined and the best one is chosen, based on
administrative distance and metric. The gateway to the best default path becomes the gateway of last
resort.
SUMMARY STEPS
1. enable
2. configure terminal
3. ip routing
4. ip route dest-prefix mask next-hop-ip-address [admin-distance] [permanent]
5. ip default-network network-number
or
ip route dest-prefix mask next-hop-ip-address
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