Chapter 5 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 Cisco IOS IP Routing Protocols Configuration Guide, Release 12.4T.
•IP Routing, page
•Default Routes, page
•Default Network, page
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,
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.
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.
| Cisco 1900 Series Hardware Installation |
|
