Default Routes And Networks - Cisco Catalyst 9500 series Configuration Manual

Default Routes and Networks

Default Routes and Networks
Information About Default Routes and Networks
A router might not be able to learn the routes to all other networks. To provide complete routing capability,
you can 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
dynamically learned or can be configured in the individual routers. Most dynamic interior routing protocols
include a mechanism for causing a smart router to generate dynamic default information that is then forwarded
to other routers.
If a router has a directly connected interface to the specified default network, the dynamic routing protocols
running on that device generate a default route. In RIP, it advertises the pseudonetwork 0.0.0.0.
A router that is generating the default for a network also might 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.
When default information is passed 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
IGRP networks, there might be several candidate networks for the system default. Cisco routers use
administrative distance and metric information to set the default route or the gateway of last resort.
If dynamic default information is not being passed to the system, candidates for the default route are specified
with the ip default-network global configuration command. If this network appears in the routing table from
any source, it is flagged as a possible choice for the default route. If the router has no interface on the default
network, but does have a path to it, the network is considered as a possible candidate, and the gateway to the
best default path becomes the gateway of last resort.
How to Configure Default Routes and Networks
Procedure
Step 1
Step 2
Step 3
Routing Configuration Guide, Cisco IOS XE Everest 16.6.x (Catalyst 9500 Switches)
186
Command or Action
configure terminal
Example:
Device# configure terminal
ip default-network network number
Example:
Device(config)# ip default-network 1
end
Example:
Device(config)# end
Configuring IP Unicast Routing
Purpose
Enters global configuration mode.
Specifies a default network.
Returns to privileged EXEC mode.