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Chapter 4: Interconnecting Multiple OSPF AreasIn Chapter 3, you learned how to configure a Cisco router within a single OSPF area. In this chapter, you will learn how to configure Cisco routers in multiple areas in an internetwork. You will also learn about the different ways to configure a Cisco router within an area.
The topics covered here are important for designing, implementing, and troubleshooting large OSPF networks. You are expected to know how to configure multi-area OSPF for the CCNP Routing exam as well. Remember that many exam questions require you to type in the answer. To be prepared, you must study this chapter carefully.
Interconnecting Multiple AreasThe basic configuration of a Cisco router with multiple areas is the same as for a single area. However, you need to determine the types of areas and how your router will fit in. This section covers the issues involved when multiple areas are present on an internetwork.
Critical InformationAreas are used within OSPF to define a group of routers and networks that belong to the same OSPF session. As you learned in Chapter 3, it is not difficult to configure a router for one OSPF area. Routing that occurs within a single OSPF area is called intra-area routing.
But what happens when you consider adding several hundred routers into a single area? Quite simply, the benefits gained through OSPF are lost, because the following problems will begin occurring:
- Frequent SPF calculations
- Very large routing tables
- Large link-state database
- Increased router CPU use
The use of multiple OSPF areas is known as hierarchical routing. This design technique allows for routing to occur between multiple OSPF areas, which is referred to as inter-area routing.
NOTE Remember that the first area that should be created in any OSPF network is area 0, also referred to as the backbone area. The creation of area 0 is a requirement of proper OSPF operation and configuration.
With multiple OSPF areas, routers must operate according to specific roles, depending on their required function. The different types of routers are as follows:
- An internal router has all of its interfaces within the same area.
- A backbone router is connected to the OSPF backbone, area 0.
- An ABR has interfaces in multiple OSPF areas.
- An ASBR has at least one interface connected to an external internetwork such as the Internet.
A router can be more than one router type at a time. For example, if a router is connected to the Internet and OSPF area 1, it is both an ASBR and an ABR.
Exam EssentialsRemember the different types of OSPF routers that can be defined.
The different types of OSPF routers that can be defined are internal router, backbone router, area border router (ABR), and autonomous system boundary router (ASBR).
Understand the difference between inter-area routing and intra-area routing.
Inter-area routing allows routing to occur between multiple OSPF areas. OSPF also performs intra-area routing, which is routing that occurs within a single OSPF area.
Key Terms and Conceptsarea border router (ABR) A router that has interfaces in multiple OSPF areas.
autonomous system boundary router (ASBR) A router that has at least one interface connected to an external internetwork such as the Internet.
backbone router A router that is connected to the OSPF backbone, area 0.
hierarchical routing Enables routing efficiency by logically separating a large internetwork into smaller areas.
intra-area routing Routing that occurs within a single OSPF area.
inter-area routing Routing that occurs between OSPF areas.
internal router A router that has all of its interfaces within the same area.
Understanding OSPF Areas, Routers, and LSAsThis section covers the various roles that routers play in a large OSPF network. It also describes the types of advertisements that are used in an OSPF network and the different types of areas that can be configured.
Critical InformationBefore you an configure multiple OSPF areas, you need to know how to define router types. Also, just as there are multiple OSPF router categories to consider, there are also multiple types of LSAs that occur in a multiple-area setup. Finally, there are several types of nonstandard areas that can be used to reduce router overhead.
As discussed earlier, routers within a multi-area OSPF network fall into different categories. To gain a better understanding of the various roles that routers can play, consider the example shown in Figure 4.1.
Figure 4. 1: Router roes...
...Starting at the core of the given network and working outward, notice that RouterA is part of area 0. As mentioned in the previous chapter, area 0 is referred to as the backbone area. Therefore, RouterA can be defined as the backbone router, because it exists (wholly or in part) in OSPF area 0.
Now consider RouterB. Notice that RouterB meets the requirement to be classified as a backbone router (because it has one or more interfaces that are part of area 0). However, unlike RouterA, RouterB is partially in area 0 and partially in area 10. Therefore, RouterB can be defined as an ABR, because it is connected to multiple OSPF areas.
Link State Advertisements
Recall that a router's link-state database is made up of LSAs. Specifically, there are five types of LSAs that are important when you have multiple OSPF areas:
Type 1 LSA Referred to as a router link advertisement (RLA), the Type 1 LSA is an advertisement sent by a router to other routers in its area. The advertisement contains the status of a router's link to the area it is connected to. If a router is connected to multiple areas, then it will send a Type 1 LSA for each of the areas it is connected to.
Type 2 LSA Referred to as a network link advertisement (NLA), the Type 2 LSA is generated by DRs. Recall that a DR is elected to represent the other routers in its network, and it has established adjacencies with each of the routers within its network. The DR uses the Type 2 LSA to send out information about the state of other routers that are part of the same network. Note that the Type 2 LSA is sent only to routers that are in the area containing the specific network.
Type 3 and Type 4 LSAs Referred to as summary link advertisements (SLAs), the Type 3 and Type 4 LSAs are generated by ABRs. These ABRs send Type 3 and Type 4 LSAs to all routers within an area. These LSAs advertise intra-area routes to the backbone area (area 0) and both intra-area and inter-area routes to nonbackbone areas....