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OSPF and IS-IS: Choosing an IGP for Large-Scale Networks by Jeff Doyle

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10.1. Extending OSPF

New capabilities are added to OSPF primarily through the addition of new LSAs. You have already encountered in Chapter 7 one example of an LSA that supports an optional capability: The NSSA (type 7) LSA. This chapter and the next three introduce other “extension” LSAs.

10.1.1. The OSPF Extensibility Problem

Two characteristics of OSPF pose problems to extensibility. The first characteristic is that when an OSPF router receives an LSA of an unknown type, it drops the LSA. The influence of this behavior is seen in NSSAs, where all routers in the area must support the NSSA option and understand type 7 LSAs. In this case, the behavior is not a problem because you want all routers in the not-so-stubby area to enforce the rule ...

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