Cisco Systems 78-11741-02 manual Creating Virtual Links, Generating a Default Route, IPC-231

Configuring OSPF

Creating Virtual Links

To have the software advertise one summary route for all redistributed routes covered by a network address and mask, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# summary-address{{ip-address mask}	Specifies an address and mask that covers
{prefix mask}} [not-advertise][tag tag]	redistributed routes, so only one summary
	route is advertised. Use the optional
	not-advertisekeyword to filter out a set of
	routes.

Creating Virtual Links

In OSPF, all areas must be connected to a backbone area. If there is a break in backbone continuity, or the backbone is purposefully partitioned, you can establish a virtual link. The two endpoints of a virtual link are ABRs. The virtual link must be configured in both routers. The configuration information in each router consists of the other virtual endpoint (the other ABR) and the nonbackbone area that the two routers have in common (called the transit area). Note that virtual links cannot be configured through stub areas.

To establish a virtual link, use the following command in router configuration mode:

Command			Purpose
Router(config-router)# area	transit-area-id virtual-link	transit-router-id	Establishes a virtual link.
[authentication {message-digest null}] [hello-interval		seconds]
[retransmit-interval seconds] [transmit-delay seconds] [dead-interval seconds]
[{authentication-key key}	message-digest-key key-idmd5 key}]

To display information about virtual links, use the show ip ospf virtual-linksEXEC command. To display the router ID of an OSPF router, use the show ip ospf EXEC command.

Generating a Default Route

You can force an ASBR to generate a default route into an OSPF routing domain. Whenever you specifically configure redistribution of routes into an OSPF routing domain, the router automatically becomes an ASBR. However, an ASBR does not, by default, generate a default route into the OSPF routing domain.

To force the ASBR to generate a default route, use the following command in router configuration mode:

Command	Purpose
Router(config-router)#default-information originate	Forces the autonomous system boundary router to
[always] [metric metric-value][metric-type type-value]	generate a default route into the OSPF routing
[route-map map-name]	domain.

For a discussion of redistribution of routes, see the “Configuring IP Routing Protocol-Independent Features” chapter.

Cisco IOS IP Configuration Guide

IPC-231