ip ospf adj command that describes a serial interface in point-to-point mode. No DR election occurs. However, the adjacency forms, allowing database description (DBD) packets to be sent during the exchange process. Notice that the neighbor relationship passes through the two-way phase and into the exchange phase. After DBD packets are sent between routers, the neighbors move into the final state, which is full adjacency. Figure displays debug ip ospf adj output illustrating the DR and BDR election process on a Fast Ethernet interface. The OSPF default behavior on a Fast Ethernet link is broadcast mode. First, the DR and BDR are selected, and then the exchange process occurs.

Content 3.6 Multiarea OSPF Operation 3.6.1 Large OSPF Network Issues The most important thing to understand in OSPF is how the topology database is built. Troubleshooting OSPF often requires analyzing the database and routing table, so a solid understanding of LSAs is essential. This lesson describes each of the common LSA types and how they form the layout of the LSDB.OSPF LSDBs are often very large. For this reason, an area hierarchical structure has been imposed that defines several router types. OSPF can usually operate within a single area; however, certain issues arise if this single area expands into hundreds of networks. If an expansion occurs, the following issues need to be addressed:

Frequent SPF algorithm calculations: In a large network, changes are inevitable. Therefore, the routers spend many CPU cycles recalculating the SPF algorithm and updating the routing table.
Large routing table: OSPF does not perform route summarization by default. If the routes are not summarized, the routing table can become very large, depending on the size of the network.
Large LSDB: Because the LSDB covers the topology of the entire network, each router must maintain an entry for every network in the area, even if every route is not selected for the routing table.

A solution to these issues is to divide the network into multiple OSPF areas. OSPF allows the separation of a large area into smaller, more manageable areas that are still able to exchange routing information. Hierarchical area routing separates a large internetwork into multiple areas. When you use this technique, interarea routing still occurs, but many of the internal routing operations, such as SPF calculations, remain within individual areas. For example, in Figure , if area 1 is having problems with a link going up and down, routers in other areas do not need to continually run their SPF calculation because they are isolated from the problem. Using multiple OSPF areas has several advantages:

Reduced frequency of SPF calculations: Since detailed route information exists within each area, it is not necessary to flood all link-state changes to all other areas. Only the routers that are affected by the change need to recalculate SPF.
Smaller routing tables: Detailed route entries for specific networks within an area can remain in the area. Instead of advertising these explicit routes outside the area, routers can be configured to summarize the routes into one or more summary addresses. Advertising these summaries reduces the number of LSAs propagated between areas but keeps all networks reachable.
Reduced LSU overhead: LSUs contain a variety of LSA types, including link-state and summary information. Rather than send an LSU about each network within an area, a router can advertise a single summarized route or small number of routes between areas, reducing the overhead associated with LSUs when they cross areas.

Interactive Media Activity Drag and Drop: Comparison of OSPF and EIGRP Features Upon completion of this activity, the student will be able to identify and compare the different features between OSPF and EIGRP.

Content 3.6 Multiarea OSPF Operation 3.6.2 OSPF LSA Types LSAs are the building blocks of the OSPF LSDB. Individually, they act as database records. In combination, they describe the entire topology of an OSPF network or area. All LSA types have 20-byte headers. One of the LSA header fields is the link-state ID. The link-state ID of the type 1 LSA is the originating router ID. Each router link is defined as an LSA type. The LSA includes a link ID field that identifies, by network number and mask, the object that this link connects to. Depending on the type, the link ID has different meanings, as described in Figure .

Content 3.6 Multiarea OSPF Operation 3.6.3 OSPF LSA Types (cont.) The following are descriptions of each type of LSA. Type 1
Every router generates router link advertisements for each area to which it belongs. A type 1 LSA describes the collective states of the directly connected links (interfaces) of the router. These LSAs are flooded only within the area in which they are originated.

Type 2
A type 2 LSA is generated for every transit broadcast and NBMA network within an area. A transit network has at least two directly attached OSPF routers. Ethernet is an example of a transit network. The DR of the network is responsible for advertising the network LSA. A type 2 network LSA lists each of the attached routers that make up the transit network, including the DR itself, as well as the subnet mask used on the link. The type 2 LSA then floods to all routers within the transit network area. Type 2 LSAs never cross an area boundary. The link-state ID for a network LSA is the IP interface address of the DR that advertises it.

Type 3
The ABR sends type 3 summary LSAs. Type 3 LSAs advertise any networks owned by an area to the rest of the areas in the OSPF autonomous system, as shown in Figure . The link-state ID is set to the network number; the mask is also advertised. By default, OSPF does not automatically summarize groups of contiguous subnets or summarize a network to its classful boundary. The network operator uses configuration commands to specify how the summarization occurs. By default, a type 3 LSA is advertised into the backbone area for every subnet defined in the originating area, which can cause significant flooding problems. Consequently, you should always consider using manual route summarization at the ABR. Summary LSAs are flooded throughout a single area only, but are regenerated by ABRs to flood into other areas. Note
By default, summary LSAs do not contain summarized routes. Type 4
A type 4 summary LSA is generated by an ABR only when an ASBR exists within an area. A type 4 LSA identifies the ASBR and provides a route to it. The link-state ID is set to the ASBR router ID. All traffic destined to an external autonomous system requires routing table knowledge of the ASBR that originated the external routes. In Figure , the ASBR sends a type 1 router LSA with an external bit (e bit) that is set to identify itself as an ASBR. When the ABR, which is identified with a border bit (b bit) in the router LSA, receives the type 1 LSA, it builds a type 4 LSA and floods it to the backbone (area 0). Subsequent ABRs regenerate a type 4 LSA to flood into their areas.

Type 5
Type 5 external LSAs describe routes to networks outside the OSPF autonomous system. Type 5 LSAs are originated by the ASBR and are flooded to the entire autonomous system. The link-state ID is the external network number. Because of the flooding scope, and depending on the number of external networks, the default lack of route summarization can be a major issue with external LSAs. Therefore, you should summarize blocks of external network numbers at the ASBR to reduce flooding problems.

Type 6
Type 6 LSAs are specialized LSAs that are used in multicast OSPF applications.

Type 7
Type 7 is an LSA type that is used in not-so-stubby areas (NSSAs). They are originated by ASBRs within NSSAs and are flooded only within the NSSA in which they originated.