5.2 Configuring OSPF Operation Across Multiple Areas
5.2.2 Flooding LSUs to multiple areas

ABRs are responsible for generating routing information about each area to which they are connected and flooding the information through the backbone area to the other areas to which they are connected. The general process for flooding is as follows:

  1. The intra-area routing processes, as discussed in the previous chapter, occur. Note that the entire intra-area must be synchronized before the ABR can begin sending summary LSAs.
  2. The ABR reviews the resulting link-state database and generates summary LSAs. 
    By default, the ABR sends summary LSAs for each network that it knows about. To reduce the number of summary LSA entries, you can configure route summarization so a single IP address can represent multiple networks. To use route summarization, your areas need to use contiguous IP addressing, as discussed in chapter 2. The better your IP address plan, the lower the number of summary LSAs entries an ABR will advertise.
  3. The summary LSAs (Type-3 and Type-4) 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 (see the main figure).
  1. After an ABR or ASBR receives summary LSAs, they add them to their link-state databases, and flood them to their local area. The internal routers then assimilate the information into their databases. 
    To reduce the number of route entries internal routers maintain, you can define the area as stub, totally stubby, or not so stubby.