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Cisco 1900 Series Hardware Installation
OL-19084-02
Chapter 6 Configuring the Router
Performing the Initial Configuration on the Router
Specifying a Default Route or Gateway of Last Resort
This section describes how to specify a default route with IP routing enabled. For alternative methods
of specifying a default route, see the
Configuring a Gateway of Last Resort Using IP Commands
tech
note.
The Cisco IOS software uses the gateway (router) of last resort if it does not have a better route for a
packet and if the destination is not a connected network. This section describes how to select a network
as a default route (a candidate route for computing the gateway of last resort). The way in which routing
protocols propagate the default route information varies for each protocol.
Configuring IP Routing and IP Protocols
For comprehensive configuration information about IP routing and IP routing protocols, see the
Cisco
IOS IP Routing Protocols Configuration Guide, Release 12.4T
on Cisco.com at the following URL
,
http://www.cisco.com/en/US/docs/ios/iproute/configuration/guide/12_4t/irp_12_4t_book.html
.
IP Routing
You can configure integrated routing and bridging (IRB) so the router can route and bridge
simultaneously. The router will act as an IP host on the network whether routing is enabled or not. To
read more about IRB see the following URL on Cisco.com,
http://www.cisco.com/en/US/tech/tk389/tk815/tk855/tsd_technology_support_sub-protocol_home.html
IP routing is automatically enabled in the Cisco IOS software. When IP routing is configured, the system
will use a configured or learned route to forward packets, including a configured default route.
Note
This task section does not apply when IP routing is disabled. To specify a default route when IP routing
is disabled, refer to the
Configuring a Gateway of Last Resort Using IP Commands
tech note on
Cisco.com.
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 pseudonetwork 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.
