OSPF LSA and Area Types

by Shabeeribm
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:
  • Router LSA (Type 1) – Contains a list of all links local to the router, and the status and “cost” of those links. Type 1 LSAs are generated by all routers in OSPF, and are flooded to all other routers within the local area. 
  • Network LSA (Type 2) – Generated by all Designated Routers in OSPF, and contains a list of all routers attached to the Designated Router. 
  • Network Summary LSA (Type 3) – Generated by all ABRs in OSPF, and contains a list of all destination networks within an area. Type 3 LSAs are sent between areas to allow inter-area communication to occur. 
  • ASBR Summary LSA (Type 4) – Generated by ABRs in OSPF, and contains a route to any ASBRs in the OSPF system. Type 4 LSAs are sent from an ABR into its local area, so that Internal routers know how to exit the Autonomous System. 
  • External LSA (Type 5) – Generated by ASBRs in OSPF, and contain routes to destination networks outside the local Autonomous System. Type 5 LSAs can also take the form of a default route to all networks outside the local AS. Type 5 LSAs are flooded to all areas in the OSPF system. 
  • Type 7 NSSA External LSAs - Used in stub areas in place of a type 5 LSA 
NOTE : LSA types 1 and 2 are found in all areas, and are never flooded outside of an area.

 Summary:
  • 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

 OSPF Area Configurations:

Stub Area
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 adjacency.
Router(Config)#router OSPF 1
Router(config-router)# area 10 stub
Totally Stubby Areas
Like stub areas, totally stubby areas do not receive type 4 or 5 LSAs from their ABRs. However, they also do not receive type 3 LSAs; all routing out of the area relies on the single default route injected by the ABR.A stub area is extended to a totally stubby area by configuring all of its ABRs with the no-summary parameter
Router(Config)#router OSPF 1
Router(config-router)# area 10 stub no-summary
To designate a normal (stub) NSSA, all routers in the area must be so configured:
Router(Config)#router OSPF 1
Router(config-router)# area 10 nssa
Type 3 LSAs will pass into and out of the area. Unlike a normal stub area, the ABR will not inject a default route into an NSSA unless explicitly configured to do so. As traffic cannot be routed to external destinations without a default route, you'll probably want to include one by appending default-information-originate
Router(config-router)# area 10 nssa default-information-originate
To expand an NSSA to function as a totally stubby area, eliminating type 3 LSAs, all of its ABRs must be configured with the no-summary parameter:
Router(Config)#router OSPF 1
Router(config-router)# area 10 nssa no-summary
Summary
  • Type 1 - Represents a router
  • Type 2 - Represents the 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
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.

Related articles

CCNP : OSPF Quick Notes
CCNP Notes : IPV6 Quick Notes
CCNP Routing: EIGRP Quick Notes
EIGRP Passive Interface
EIGRP Stub
EIGRP Configuration and Troubleshooting commands
Basic notes on BGP (border gateway protocol)
Quick reminder about HSRP
EIGRP Authentication
EIGRP summarization

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