Cisco Systems 78-11741-02 manual Specifying Default Routes, Specifying a Default Network, IPC-365

Configuring IP Routing Protocol-Independent Features

Specifying Default Routes

Table 9	Dynamic Routing Protocol Default Administrative Distances (continued)
Route Source		Default Distance
Enhanced IGRP (EIGRP) summary route		5
Exterior Border Gateway Protocol (BGP)		20
Internal EIGRP		90
IGRP		100
OSPF		110
IS-IS		115
RIP		120
EIGRP external route		170
Interior BGP		200
Unknown		255

Static routes that point to an interface will be advertised via RIP, IGRP, and other dynamic routing protocols, regardless of whether redistribute static router configuration commands were specified for those routing protocols. These static routes are advertised because static routes that point to an interface are considered in the routing table to be connected and hence lose their static nature. However, if you define a static route to an interface that is not one of the networks defined in a network command, no dynamic routing protocols will advertise the route unless a redistribute static command is specified for these protocols.

When an interface goes down, all static routes through that interface are removed from the IP routing table. Also, when the software can no longer find a valid next hop for the address specified as the address of the forwarding router in a static route, the static route is removed from the IP routing table.

Specifying 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.

Specifying a Default Network

If a router has a directly connected interface onto the specified default network, the dynamic routing protocols running on that device 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 IOS IP Configuration Guide

IPC-365