OSPF- Area Types-Area0, Stub (Part-5)

OSPF (Open Shortest Path First) OSPF organizes the network into areas to improve scalability, efficiency, and manageability of Large and reduce the complexity of routing information exchange. OSPF area type configuration depends on the specific network design and requirements. Different areas provide different levels of control and routing information exchange, and they can be used to optimize the routing process and reduce the size of routing tables.

Some key reasons for using areas in OSPF are to reduce LSA Flooding, Improved Convergence, Control and Security, Hierarchical Topology, less Memory use and Flexibility.

 

OSPF Area Types
OSPF Area Types

Types of OSPF Areas

  1. Backbone Area
  2. Standard Area
  3. Stub Area
  4. Totally Stubby Area
  5. Not So Stubby Area (NSSA)
  6. Totally Not So Stubby Area (TNSSA)

Area Types with Examples

Backbone Area (Area-0)

It is also known as Area-0. It is mandatory that all other areas must have a direct connection to Area 0, Area 0 is also called the transit area to connect all other areas. It is required to perform the OSPF function. The router connected to the Backbone Area is called ABR or ASBR. If any area can not connect with Area-0 or backbone area then it must be connected via Virtual Link.

Standard Area (Area-1)
  • It is a normal OSPF area
  • Routers that are part of standard Area will share LSA-1 (Router) and LSA-2 (Network).
  • It accepts LSA-3 (Network Summary LSA) from ABR to reach all other Areas’ routers.
  • It accepts LSA-4 (ASBR Summary) from ABR to reach external networks.
Stub Area
  • ABR prevents external routes or LSA-4 and LSA-5 from flooding into an area.
  • Like Standard areas, Stub area routers will share Type 1 and Type 2 LSAs to build their topology tables.
  • ABR generates Type 3 LSAs with Default Route to reach other areas and External AS.
  • Stub areas will not accept Type 4 or Type 5 LSAs, detailing routes to external networks.
  • The purpose of Stub areas is to limit the number of LSAs flooded into the area, to conserve bandwidth and router CPUs.
  • The Stub’s ABR will automatically inject a default route into the Stub area so that those routers can reach the external networks.
  • The ABR will be the next hop for the default route.
  • Area 0 (the backbone area) cannot be made a stub area.
  • No ASBRs are allowed in a Stub area, because autonomous system boundary routers inject external routes, do not make any area containing an ASBR a stub area
  • The area # stub command must be configured on all routers in the Stub area

Configuration of stub areas is relatively simple:

R3#

router ospf 10

log-adjacency-changes

area 1 stub

network 20.1.1.0 0.0.0.255 area 0

network 10.1.2.0 0.0.0.255 area 1

Below is a command where we can check Stub area details on ABR (R3).

 

Total Stubby Area
  • Prevents both inter-area and external routes from flooding into an area.
  • Like Standard and Stub areas, Totally Stubby area routers will share Type 1 and Type 2 LSAs to build their topology tables.
  • Totally Stubby areas will not accept Type 3 LSAs to other areas.
  • Totally Stubby areas will also not accept Type 4 or Type 5 LSAs, detailing routes to external networks.
  • The Stub’s ABR will instead automatically inject a default route into the Totally Stubby area.
  • The ABR will be the next hop for the default route.
  • The area 1 stub no-summary command is configured only on the ABR of the Totally Stubby area.
  • Other routers within the area are configured with the area 1 stub command.
  • No ASBRs are allowed in a Totally Stubby area.

Configuration of totally stubby areas:

R5(config)#router ospf 10

R5(config-router)#router ospf 10

R5(config-router)# area 2 stub no-summary

R5(config-router)# network 20.1.2.0 0.0.0.255 area 0

R5(config-router)# network 30.1.3.0 0.0.0.255 area 2

R5(config-router) # network 30.1.2.0 0.0.0.255 area 2

Not So Stubby Area (NSSA)
  • Like a stub area, a not-so-stubby area does not propagate Type 5 LSAs, which means an ASBR cannot be part of a stub area.
  • To import external routes, Instead of the ASBR sending out Type 5 LSAs, it will send out Type 7 NSSA External LSAs.
  • The Type 7 LSAs cannot be advertised in another OSPF area. So, ABR for the NSSA receives the Type 7 LSA and translates it into a Type 5 LSA.
  • The Type 5 LSA is then allowed to be flooded throughout the OSPF autonomous system.
  • Like Standard and Stub areas, NSSA area routers will share Type 1 and Type 2 LSAs to build their topology tables.
  • NSSA areas will also accept Network Summary (Type 3) LSAs, which contain the routes to reach networks in all other areas.
  • NSSA areas will not accept Type 4 or Type 5 LSAs.

Below is the command to configure NSSA on ABR

Router(config-router) # area 1 nssa

Totally Not So Stubby Area (TNSSA)
  • Like Totally Stubby area; prevents both inter-area and external routes from flooding into an area, unless those external routes originated from an ASBR within the NSSA area.
  • Like Standard and Stub areas, TNSSA area routers will share Type 1 and Type 2 LSAs to build their topology tables.
  • TNSSA areas will not accept Type 3 LSAs to other areas.
  • TNSSA areas will not accept Type 4 or Type 5 LSAs, detailing routes to external networks.
  • If an ASBR exists within the TNSSA area, that ASBR will generate Type 7 LSAs.

Configuration of TNSSA:

Router(config-router) # area 1 nssa no-summary

 

Read more OSPF-Part-1  –https://techblog.kbrosistechnologies.com/what-is-ospf-basics-routing-protocols-link-state-routing-protocol/

Read more OSPF-Part-2- https://techblog.kbrosistechnologies.com/what-is-ospf-dr-bdr-network-type-neighbour-state-routing-protocol/

Read more OSPF-Part-3-https://techblog.kbrosistechnologies.com/ospf-part-3-advance-router-types-abr-asbr-area-0-backbone-area-virtual-link/

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