OSPF Area Types

• Stub Area (SA)

  • Here, an ABR blocks flooding of Type 4 and 5 LSA’s, and instead, generates a Type 3 LSA with the default route for all network external to the AS.
  • The ABR then floods that and any intra-area Type 3 LSA’s to all internal routers in the (stub) area.
  • So, all internal routers know that the ABR is the default gateway for traffic external to the stub area.

• Totally-Stub-Area (TSA)

  • Don’t propagate Type 3, 4 and 5 LSA’s, except for one Type 3 LSA that advertises the default route of the area.
  • The only way a router in the TSA can reach the external AS is through the ABR.
  • This is a purely Cisco-specific function, and may not be available on other vendors’ equipment.

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