Types of OSPF areas and Routers

OSPF areas and router types

•  OSPF relies on several types of Link State Advertisements (LSAs) to communicate link state information between neighbors. A brief review of the most applicable LSA types:
•  
• Type 1 - Represents a router
• Type 2 - Represents the pseudonode (designated router) for a multiaccess link
• Type 3 - A network link summary (internal route)
• Type 4 - Represents an ASBR
• Type 5 - A route external to the OSPF domain
• Type 7 - Used in stub areas in place of a type 5 LSA

LSA types 1 and 2 are found in all areas, and are never flooded outside of an area. Whether the other types of LSAs are advertised within an area depends on the area type, and there are many:

• Backbone area (area 0) LSA Type 1:Router LSA
• Standard area  LSA type 2: Network LSA (DR generated)
  
• Stub area  LSA Type 3:Summary LSA (ABR summary route)
  
• Totally stubby area LSA Type 4: Summary LSA (ASBR Location)
  
• Not-so-stubby area (NSSA)  LSA Type 5: External LSA (ASBR Summary roue)

For an area to become a stub, all routers belonging to it must be configured to operate as such. Stub routers and non-stub routers will not form adjacencies.

Router(config-router)# area 10 stub 

A stub area is extended to a totally stubby area by configuring all of its ABRs with the no-summary parameter:
Router(config-router)# area 10 stub no-summary 

An NSSA can function as either a stub or totally stubby area. To designate a normal (stub) NSSA, all routers in the area must be so configured:

Router(config-router)# area 10 nssa

• Standard areas can contain LSAs of type 1, 2, 3, 4, and 5, and may contain an ASBR. The backbone is considered a standard area.
• Stub areas can contain type 1, 2, and 3 LSAs. A default route is substituted for external routes.
• Totally stubby areas can only contain type 1 and 2 LSAs, and a single type 3 LSA. The type 3 LSA describes a default route, substituted for all external and inter-area routes.
• Not-so-stubby areas implement stub or totally stubby functionality yet contain an ASBR. Type 7 LSAs generated by the ASBR are converted to type 5 by ABRs to be flooded to the rest of the OSPF domain

There are four types of OSPF routers which are determined by a router’s function and/or location within anOSPF area:

Internal (IR) all OSPF interfaces must belong to the same OSPF area. 

Backbone–at least one OSPF interface must belong to area 0 (backbone area)

Area Border Router (ABR)at least one OSPF interface must belong to area 0 (backbone area) and at least one OSPF interface must belong to a non-backbone (area 0) area.

Autonomous System Boundery Router (ASBR)–an OSPF router that performs route injection (redistribution)from another route source (RIP, EIGRP, ISIS, BGP, another OSPF process, etc.). 

You can use the show ip ospfEXEC command to identify which OSPF type(s) your device is currently configured

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