class D multicast address 224.0.0.5. The hello packets contain neighbor parameters on which the two routers must agree before they can become neighbors. Parameters include the router ID and area ID of the originating router, authentication settings, timer settings, router priority, and designated router (DR) and backup designated router (BDR) information.

Routers declare the neighbor up when the exchange is complete.
After two routers establish neighbor adjacency using the hello packets, they synchronize their LSDBs by exchanging LSAs and confirming the receipt of LSAs from the adjacent router. The two neighboring routers now recognize that they have synchronized their LSDBs with each other. For OSPF, the routers are now in full adjacency state.
If necessary, the routers forward any new LSAs to other neighboring routers, ensuring complete synchronization of link-state information inside the area.

Forming OSPF adjacencies also depends on the network type. For example, two OSPF routers on a point-to-point serial link form a full adjacency with each other once they have agreed on the specified parameters. On broadcast multiaccess networks such as Ethernet, a DR and BDR election needs to take place. The OSPF routers on LAN segments elect one router as the DR and another as the BDR. All other routers on that particular LAN segment form full adjacencies with these two routers and pass LSAs only to them. The DR forwards the updates from one neighbor on the LAN to all other neighbors on that LAN.

Content 3.1 Review of OSPF Fundamentals and Features 3.1.5 OSPF Area Structure In small networks, the web of router links is not complex, and paths to individual destinations are easily deduced. However, in large networks, the resulting web is highly complex, and the number of potential paths to each destination is large. Therefore, the SPF calculations comparing all these possible routes can be very complex and can take significant time. The CPU and memory resources on an OSPF router can also be overburdened in a heavily populated OSPF network. To reduce the SPF calculations, link-state routing protocols can partition networks into sub-domains called areas. An area is a logical collection of OSPF networks, routers, and links that have the same area identification. A router within an area maintains a topological database for the area to which it belongs. Therefore the number of routers and LSAs that flood the area are also smaller. This keeps the LSDB for an area smaller and, as a result, the SPF calculations are less resource intensive. In addition, the router does not have detailed information about network topology outside of its area, thereby reducing the size of its database. Areas limit the scope of route information distribution and reduce the number of routes to propagate. The LSDBs of routers within the same area must be synchronized and be exactly the same. However, route summarization and filtering is possible between different areas. Link-state routing protocols use a two-layer area hierarchy:

Transit area: Interconnects OSPF area types within a single domain. Generally, end users are not found within a transit area. OSPF area 0, also known as the backbone area, is a transit area.
Regular area: Connects users and resources. Regular areas are usually set up along functional or geographical groupings. By default, a regular area does not allow traffic from another area to use its links to reach other areas. All traffic from other areas must cross a transit area. A regular area, or nonbackbone area, can have a number of subtypes, including a standard area, stub area, totally stubby area, and not-so-stubby area (NSSA).

OSPF enforces this rigid two-layer area hierarchy. The underlying physical connectivity of the network must map to the two-layer area structure, with all nonbackbone areas attaching directly to area 0. In link-state routing protocols, all routers must keep a copy of the LSDB; the more OSPF routers, the larger the LSDB. It can be advantageous to have all information in all routers, but this approach does not scale to large network sizes. OSPF solves this problem by breaking up a large network into multiple areas. Routers inside an area maintain detailed information about the links in their local area and only general or summary information about routers and links in other areas. When a router or link fails, that information is flooded along adjacencies only to the routers in the local area. SPF calculations are also reduced, since detailed route information and local changes are kept within each area. By maintaining a hierarchical structure and limiting the number of routers in an area, an OSPF autonomous system can scale to very large sizes. In OSPF, all areas must connect directly to area 0, which is the backbone. In the figure, notice that links between area 1, 2, and 3 routers are not allowed. All traffic moving from one area to another area must traverse the backbone area. This traffic is referred to as interarea traffic. The optimal number of routers per area varies based on factors such as network stability. Web Links Designing Large-Scale Internetworks
http://www.cisco.com/en/US/tech/tk1330/
technologies_design_guide_chapter09186
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Content 3.1 Review of OSPF Fundamentals and Features 3.1.6 OSPF Router Types OSPF routers are categorized based on the function they perform in the routing domain. The four different types of OSPF routers are:

Internal routers: Routers that have all their interfaces in the same area and have identical LSDBs.
Backbone routers: Routers that sit on the perimeter of the backbone area and have at least one interface connected to area 0. Backbone routers maintain OSPF routing information using the same procedures and algorithms as internal routers.
Area border routers: Routers that have interfaces attached to multiple areas, maintain separate LSDBs for each area to which they connect, and route traffic destined to or arriving from other areas. Area border routers (ABRs) are exit points for the area, which means that routing information destined for another area can get there only via the ABR of the local area.

ABRs can be configured to summarize the routing information from the LSDBs of their attached areas. ABRs distribute the routing information into the backbone. The backbone routers then forward the information to the other ABRs. In a multiarea network, an area can have one or more ABRs.

Autonomous System Boundary Routers: Routers that have at least one interface attached to an external internetwork (another autonomous system), such as a non-OSPF network. Autonomous system boundary routers (ASBRs) can import non-OSPF network information to the OSPF network and vice versa; this process is called route redistribution.

A router can exist as more than one router type. For example, if a router interconnects to area 0 and area 1, in addition to a non-OSPF network, it is both an ABR and an ASBR. A router has a separate LSDB for each area to which it connects; therefore, an ABR could have one LSDB for area 0 and another LSDB for another area in which it participates. Two routers belonging to the same area maintain identical LSDBs for that area. An LSDB is synchronized between pairs of adjacent routers. On broadcast