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| 6.5 |  | Multiarea OSPF Operation |
| 6.5.5 |  | Flooding LSUs to multiple areas |

An ABR is responsible for generating routing information about each area to which it is connected. Then it floods the information through the backbone area to the other areas to which the backbone is connected. The general process for flooding follows these steps:

1. The routing processes occur within the area, as discussed in Module 4. The entire area must be synchronized before the ABR can begin sending summary LSAs to other areas.
2. The ABR reviews the resulting link-state database and generates summary LSAs, using Type 3 or Type 4. By default, the ABR sends summary LSAs for each network that it knows about. To reduce the number of summary LSA entries, configure route summarization so that a single IP address can represent multiple networks. To use route summarization, the areas need to use contiguous IP addressing, as discussed in Module 2. The better this IP address plan is, the fewer the number of summary LSA entries an ABR will advertise.
3. The summary LSAs are placed in an LSU and distributed through all ABR interfaces, with the following exceptions:

> - If the interface is connected to a neighboring router that is in a state below the exchange state, then the summary LSA is not forwarded.
> - If the interface is connected to a totally stubby area, then the summary LSA is not forwarded.
> - If the summary LSA includes a Type 5 (external) route and the interface is connected to a stub or totally stubby area, then the LSA is not sent to that area.

4. After an ABR or ASBR receives summary LSAs, it adds them to its link-state databases and floods them to the local area. The internal routers then assimilate the information into their databases. Remember that OSPF enables the ability to configure different area types so that the number of route entries that internal routers maintain can be reduced. To minimize routing information, define the area as a stub area, a totally stubby area, or an NSSA.