Cisco Systems OL-4580-01 manual Area Border Routers, Stub Area, Autonomous System Boundary Routers

Chapter 3 Configuring Open Shortest Path First (OSPF)

OSPF Overview

Area Border Routers

ABRs have multiple interfaces that connect directly to networks in two or more areas. An ABR runs a separate copy of the OSPF algorithm and maintains separate routing data for each area that is connected to it, including the backbone area.

ABRs also send configuration summaries for their attached areas to the backbone area, which distributes this information to other OSPF areas in the autonomous system. In Figure 3-1, there are two ABRs. ABR 1 interfaces area 1.1.1.1 to the backbone area. ABR 2 interfaces the backbone area to area 1.1.1.2, a stub area.

Note ABRs are always backbone routers. You must configure ABRs to the backbone area.

Stub Area

A stub area is an area that does not accept or distribute detailed network information external to the area. A stub area has only one router that interfaces the area to the rest of the AS. The ABR attached to the stub area advertises a single default external route into the area. Routers within a stub area use this route for destinations outside the autonomous system, as well as for inter-area routes. This relationship conserves LSA database space that would otherwise be used to store external LSAs flooded into the area. As shown in Figure 3-1, area 1.1.1.2 is a stub area that is reached only through ABR 2.

Autonomous System Boundary Routers

ASBRs provide connectivity from one autonomous system to another system. ASBRs exchange their autonomous system routing information with boundary routers in other autonomous systems. Every router inside an autonomous system knows how to reach the boundary routers for its autonomous system.

ASBRs can import external routing information from other protocols like RIP and redistribute them as AS-external LSAs to the OSPF network. If the CSS is an ASBR, you can configure it to advertise VIP addresses for content as AS external routes. In this way, ASBRs flood information about external networks to routers within the OSPF network.

	Cisco Content Services Switch Routing and Bridging Configuration Guide
OL-4580-01		3-5