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| 6.5 |  | Multiarea OSPF Operation |
| 6.5.4 |  | Configuring OSPF operation across multiple areas |

Internal routers, ABRs, ASBRs, and backbone routers each play a role in communicating OSPF routing information in a multiarea network. This section summarizes how the different types of OSPF routers flood information and how they build their routing tables when operating within a multiarea environment.

It was seen that a packet destined for a network within an area is merely forwarded from one internal router to another until it reaches the destination network.

However, what if a packet must traverse multiple areas as shown in the figure?

In this case, the packet must exit Area 1 by way of ABR1. ABR1 then sends the packet through the backbone area to ABR2. Finally, ABR2 can forward the packet to an internal router in Area 50. The internal router then delivers the message to the appropriate host on that network.

For the OSPF routers in this example to make these routing decisions, they must build sufficient routing tables by exchanging LSUs. The LSU exchange process within a single OSPF area relies on just two LSA types, Type 1 and Type 2. To distribute routing information to multiple areas efficiently, Type 3 and Type 4 LSAs must be used by ABRs. The following sections describe how LSUs containing the various LSA types are flooded to multiple areas, and how OSPF routers use this information to update their routing tables.