An ABR is responsible for generating
routing information about each area to which it is connected and
flooding 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 Chapter 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 (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, you
can configure route summarization so that 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 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 you to configure different area types so that you
can reduce the number of route entries that internal routers
maintain. To minimize routing information, you can define the
area as a stub area, a totally stubby area, or an NSSA.
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