Specifying A Default Route Or Gateway Of Last Resort; Configuring Ip Routing And Ip Protocols; Default Routes; Default Network - Cisco WS-C2950-24 Installation Manual

Chapter 4 Configuring 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
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.
This task section does not apply when IP routing is disabled. To specify a default route when IP routing
Note
is disabled, refer to the
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.
OL-18712-01
Configuring a Gateway of Last Resort Using IP Commands
Configuring a Gateway of Last Resort Using IP Commands
Performing the Initial Configuration on the Router
Cisco 2900 and 3900 Series Hardware Installation
tech note.
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