Cisco Systems 3750E Specifying Default Routes and Networks, Route Source Default Distance, 38-79

Chapter 38 Configuring IP Unicast Routing

Configuring Protocol-Independent Features

Table 38-14	Dynamic Routing Protocol Default Administrative Distances
Route Source		Default Distance
Connected interface		0
Static route		1
Enhanced IRGP summary route		5
External BGP		20
Internal Enhanced IGRP		90
IGRP		100
OSPF		110
Internal BGP		200
Unknown		225

Static routes that point to an interface are advertised through RIP, IGRP, and other dynamic routing protocols, whether or not static redistribute 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 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. When the software can no longer find a valid next hop for the address specified as the forwarding router's address in a static route, the static route is also removed from the IP routing table.

Specifying 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.

Beginning in privileged EXEC mode, follow these steps to define a static route to a network as the static default route:

	Command	Purpose
Step 1
	configure terminal	Enter global configuration mode.
Step 2
	ip default-network network number	Specify a default network.
		Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02			38-79