OSPF Area Types and LSA Propagation

The idea of this post is to show the LSA propagation manner depending on area type.

OSPF routing protocol has hierarchical network topology that use concept of area. OSPF area reduces the protocol’s impact on CPU and memory. Resources can be saved by blocking the propagation of some type of LSA to specific areas. Lets recall the LSA types to see how they are propagated between areas.

LSA Types

  • LSA 1 – O, Router LSA contains all Link IDs – network, generated by every router and is local to the area
  • LSA 2 – O, Network LSA contains all routers attached to the segment, generated by DR and is local to the area
  • LSA 3 – O IA, Network Summary LSA describes network from another area, generated by ABR and is propagated between areas
  • LSA 4 – O IA, Summary ASB Link States, generated by the ABR and is propagated between areas
  • LSA 5 – O E1, O E2, External Link States, generated by ASBR and is propagated between areas
  • LSA 7 – O N1, O N2, NSSA External Link States, generated by ASBR into NSSA area and is propagated into area 0 as E1 or E2

Knowing the LSA types we have to state one important OSPF protocol rule that allows devides entire OSPF domain to smaler areas. A OSPF router must share an identical link-state database only with the other routers in its area, not with the entire OSPF domain. Does it mean for example that internal router in area 2 does not need to know about LSA generated by the internal router in area 1 or by ASBR in area 3 that redistributes routes from another routing domain.

Basically OSPF area types can be devided into three types: Normal/Standard Area, Stub Area and Not-So-Stubby. The difference is that Standard gets all LSA types but Stub and Not-So-Stubby Areas have some LSA limitation. It’s worth to mention that OSPF have few variations of Stub and Not-So-Stubby. Below you can find OSPF domain diagram that shows which LSA type is or is not propagated into specific area type and explanation of each area type.

Here you are the basic keyword and rule that help you understand the concept:

Words “Totally Stubby” = “no-summary” keyword in the area type command definition = no LSA Type 3,4 and 5 propagation into area instead ABR produce default route as Intra Area LSA (O*IA 0.0.0.0/0) into Totally Stubby Area.

 

 

Stub Area – area <area> stub

Allows propagation of LSA type 1,2 and 3  additionally with default route as Intra Area LSA format (O*IA 0.0.0.0/0)

Blocks propagation of LSA type 4 and 5

  • Stub area can not be a transit area for Virtual link, but a GRE tunnel can be used instead
  • Stub area can not have an ASBR
  • The Backbone area can not be configured as a Stub area
  • No LSA type 4 and 5 (E1 or E2) is allowed in a Stub area, but the routers in the Stub Area can connect to the External routes via the default route that is injected into the area by the ABR
  • Every router and the ABR of that area should have “area <area> stub” command
  • By default, the cost of the default route is 1
  • The cost of the default route can be changed by “area <area> default-cost <cost>”

Totally Stubby – area <area> stub no-summary  

Allows propagation of LSA type 1 and 2 additionally with default route as Intra Area LSA format (O*IA 0.0.0.0/0)

Blocks propagation of LSA type 3,4 and 5

  • This area has the same features as standard Stub Area
  • Only ABR has to be configured with the “no-summary” keyword in area definition
  • Intra area routers in the Totally Stubby area should have “area <area> stub” command only

Not-So-Stubby – area <area> nssa

Allows propagation of LSA 1, 2, 3 and 7

Blocks propagation of LSA type 3,4 and 5, no default route

  • All Intra Area routers in the Not-So-Stubby area should have “area <area> nssa” configured
  • Not-So-Stubby area can not be a transit area for Virtual link, but a GRE tunnel can be used instead
  • No LSA type 3,4 and 5 (E1 or E2) is allowed in a Not-So-Stubby area, but external routes can be injected into area and appear as LSA type 7, then translated on the ABR to LSA type 5
  • Not-So-Stubby area can have an ASBR
  • The backbone area can not be configured as a Not-So-Stubby area

Totally Stubby Not-So-Stubby – area <area> nssa no-summary

Allows propagation of LSA 1 and 2 additionally with default route as Intra Area LSA format (O*IA 0.0.0.0/0)

Blocks propagation of LSA type 3,4 and 5

  • This area has the same features as standard Not-So-Stubby the only one difference is that default route as Intra Area LSA format (O*IA 0.0.0.0/0) is produced

Not-So-Stubby – area <area> nssa default-information-originate

Allows propagation of LSA type 1, 2, 3 and 7 additionally with default route as LSA type 7 Eternal 2  LSA format (O*N2 0.0.0.0/0)

Blocks propagation of LSA type 4 and 5

  • This area has the same features as standard Not-So-Stubby the only one difference is that default route as LSA type 7 Eternal 2  LSA format (O*N2 0.0.0.0/0) is produced

Totally Stubby Not-So-Stubby – area <area> nssa no-summary no-redistribution

Allows propagation of LSA 1 and 2 additionally with default route as Intra Area LSA format (O*IA 0.0.0.0/0)

Blocks propagation of LSA type 3,4 and 5

  • This area has the same features as standard Not-So-Stubby the only one difference is that prefixes that have been redistributed on the ABR will not be propagated into area

http://www.netcontractor.pl/blog/?p=451

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