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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:
- 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.
- 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.
- 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).
- 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.
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