Router Types

Given the hierarchical nature of the OSPF network, you will see routers operating within an area, routers connecting areas, and routers connecting the organization or autonomous system to the outside world. Each of these routers will have a different set of responsibilities, depending on their position and functionality within the OSPF hierarchical design.

The following list identifies the different OSPF routers:

• Internal router—Within an area, the functionality of the router is straightforward. It is responsible for maintaining a current and accurate database of every subnet within the area. It is also responsible for forwarding data to other networks by the shortest path. Flooding of routing updates is confined to the area. All interfaces on this router are within the same area.

• Backbone router—The design rules for OSPF require that all the areas be connected through a single area, known as the backbone area or Area 0. Area 0 is also known as Area 0.0.0.0 on other routers. A router within this area is referred to as a backbone router. It may also be an internal router or an Area Border Router.

• Area Border Router (ABR)—This router is responsible for connecting two or more areas. It holds a full topological database for each area to which it is connected and sends LSA updates between the areas. These LSA updates are summary updates of the subnets within an area. It is at the area border that summarization should be configured for OSPF because this is where the LSAs make use of the reduced routing updates to minimize the routing overhead on both the network and the routers.

• Autonomous System Boundary Router (ASBR)—To connect to the outside world or to any other routing protocol, you need to leave the OSPF domain. OSPF is an interior routing protocol or Interior Gateway Protocol (IGP); gateway is an older term for a router. The router configured for this duty is the ASBR. If there is any redistribution between other protocols to OSPF on a router, it will be an ASBR. Although you can place this router anywhere in the OSPF hierarchical design, it should reside in the backbone area. Because any traffic leaving the OSPF domain will also likely leave the router's area, it makes sense to place the ASBR in a central location that all traffic leaving its area must traverse.

Figure 6-2 shows how the different router types are interrelated.

Figure 6-2 Router Definitions for OSPF

Figure 6-2 Router Definitions for OSPF

Figure 6-3 shows the connectivity and functionality of the different areas.

The routers will send out routing updates and other network information through LSAs. The function or type of router will determine the LSAs that are sent.

Figure 6-3 The Different Types of OSPF Areas and LSA Propagation

Figure 6-3 The Different Types of OSPF Areas and LSA Propagation

Stub area Totally

(SA) stubby area

Stub area Totally

(SA) stubby area

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