OSPF, propagating a default route

This is a core TI.

Two important configurations are established in this TI. A configured default route is used by a router to generate a gateway of last resort. The static default route configuration syntax uses the network 0.0.0.0 address and a subnet mask 0.0.0.0.

Router(config)#ip route 0.0.0.0 0.0.0.0 S1

Router(config)#ip route 0.0.0.0 0.0.0.0 192.168.1.3

Point out that an interface can be used as well as a next hop address.

This is referred to as the quad-zero route, and any network address is matched using the following rule. The network gateway is determined by ANDing the packet destination with the subnet mask. The following configuration statement will propagate this route to all the routers in a normal OSPF area:

Router(config-router)#default-information originate

All routers in the OSPF area will learn a default route provided that the interface of the border router to the default gateway is up.

The practical lab and e-Lab in this TI will enable students to propagate default route information in an OSPF domain.

This page will teach students how to configure a default route for an OSPF router.

OSPF routing ensures loop-free paths to every network in the domain. To reach networks outside the domain, either OSPF must know about the network or OSPF must have a default route. To have an entry for every network in the world would require enormous resources for each router.

A practical alternative is to add a default route to the OSPF router connected to the outside network. This route can be redistributed to each router in  the AS through normal OSPF updates.

A configured default route is used by a router to generate a gateway of last resort. The static default route configuration syntax uses the network 0.0.0.0 address and a subnet mask 0.0.0.0:

Router(config)#ip route 0.0.0.0 0.0.0.0 [interface | next-hop address ]

This is referred to as the quad-zero route, and any network address is matched using the following rule. The network gateway is determined by ANDing the packet destination with the subnet mask.

The following configuration statement will propagate this route to all the routers in a normal OSPF area:

Router(config-router)#default-information originate

All routers in the OSPF area will learn a default route provided that the interface of the border router to the default gateway is active.

The Lab Activities will help students configure an OSFP network and then set up a default route.

The next page will discuss some important considerations for OSPF routers.

Lab Exercise: Propagating Default Routes in an OSPF Domain

In this lab, students will configure the OSPF network so that all hosts in the OSPF area can connect to outside networks.

e-Lab Activity: Propagate Default Route Information in an OSPF Domain

In this lab, the student will configure the OSPF network so that all hosts in the OSPF area can connect to outside networks.

default-information originate (OSPF)