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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:
- 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.
- 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.
- 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.
- 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.
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