Type 3 LSA Filtering
ABRs, by definition, connect to the backbone area and at least one other area. ABRs, as a fundamental part of their role, create and flood Type 3 Summary LSAs into one area to represent the subnets in the other areas connected to that ABR. Type 3 LSA filtering tells the ABR to filter the advertisement of these Type 3 LSAs.
For example, consider Figure 9-1, which shows a generalized design with two ABR routers. The figure focuses on three subnets in area 0 for which each ABR would normally create and flood a Type 3 Summary LSA into area 1. However, in this case, the engineer has made the following choices:
On ABR1, filter subnet ...
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