CCNP ROUTE 2.0: OSPF Area Types

A Stub area is a special OSPF area that can be configured to filter out external or inter-area routes from entering the stub area, a default route is generated in its place. Configuring a stub area reduces the size of the LSDB/routing table, reduces the amount of SPF calculations and reduces the CPU/Memory utilisation of the routers inside that area.

Standard (Normal) Area

  • Allows Type 3 and Type 4 Summary LSAs
  • Allows Type 5 External LSAs

Stub Area

  • No Type 5 External LSAs (external routes)
  • No ASBR allowed
  • Default route and inter-area routes advertised

Totally Stubby

  • No Type 3 Summary LSA (inter-area routes)
  • No Type 5 External LSA (external routes)
  • No ASBR allowed
  • Only a default route advertised in routing table

Not So Stubby Area (NSSA)

  • Variant of a stub area, but allows ASBR
  • No Type 5 External LSAs allowed, converts it’s Type 5 LSAs into a Type 7 LSA
  • Only inter-area routes advertised
  • NO default route advertised as default, unlike a stub area. Can use “area x nssa default-information originate” command to create a default route.
  • Uses N1 and N2 routes in the routing table

Totally Not So Stubby Area (NSSA)

  • No Type 3 Summary LSA (inter-area routes)
  • No Type 5 External LSA (external routes)
  • ASBR allowed
  • Only a default route advertised in routing table


Configure Stub Areas

In this example we have inter-area and external routes setup and distributed to all routers. Router R3 will be configured as a stub

The screenshot below is of the routing table of router R3 before configuring the router as a stub. Notice inter-area routes (O IA) and external routes (O E1) in the routing table.

Stub Area

R2 (config)# router ospf 1
R2 (config-router)# area 1 stub

R3 (config)# router ospf 1
R3 (config-router)# area 1 stub

After entering the commands on both routers R2 and R3 the adjacency will drop and re-establish. Once the routing table is up to date you will notice there are now no longer any external routes (O E1 or O E2), instead a default inter-area route 0.0.0.0/0 exists.



Totally Stubby Area

R2 (config)# router ospf 1
R2 (config-router)# area 1 stub no-summary

R3 (config)# router ospf 1
R3 (config-router)# area 1 stub

To configure the area as “Totally Stubby” the command “area x stub no-summary” needs configuring on the ABR only. As can be seen in the screenshot below no inter-area or external routes exist on the router except the default route 0.0.0.0/0.

 

Configure NSSA Areas

In the example below EIGRP is redistributed to multiple routers on the network. Area 1 will be configured as an NSSA.

Before NSSA is configured on area 1 (R2 and R3) the routing table on R3 has the OSPF inter-area routes from R1 (172.16.0.0 – 172.16.3.0), external routes from R6 (192.168.10.0 & 192.168.11.0), external routes from R4 (10.10.0.0 – 10.10.3.0) as well as the EIGRP routes from R5 (192.168.20.0 & 192.168.21.0).

NSSA

Configuration of NSSA on area in OSPF will filter Type 5 External LSAs from entering the area.

R2 (config)# router ospf 1
R2 (config-router)# area 1 nssa

R3 (config)# router ospf 1
R3 (config-router)# area 1 nssa

Post configuration of NSSA on area 1 you will now notice that the External routes from R6 (192.168.10.0 & 192.168.11.0) as expected are no longer in the routing table BUT the External routes from R4 remain, as “O N1” routes instead of “O E1” as before. This scenario is because router R2 is an ASBR and an ABR which generates Type 5 and Type 7 LSAs, NSSA only filters Type 5 LSAs.


In this instance only, the option “no-redistribution” must be used in conjunction with “area x nssa” command. Without the “no-redistribution” option the routes learned from R4 would be in the LSDB of R3 as Type 7 LSAs.

R2 (config)# router ospf 1
R2 (config-router)# area 1 nssa no-redistribution

R3 (config)# router ospf 1
R3 (config-router)# area 1 nssa

Once the no-redistribution option is configured on R2, the Type 7’s are no longer distributed from outside of the area. The only Type 7s that remain locally originated.


The routing table now no longer has an external (O N1 or O N2) routes. It does however has inter-area routes (O IA) but NO default route.

Totally Not So Stubby Area

Totally NSSA will filter out the Type 3 LSAs and create a default route.

R2 (config)# router ospf 1
R2 (config-router)# area 1 nssa no-summary

R3 (config)# router ospf 1
R3 (config-router)# area 1 nssa

 

Only the EIGRP routes remain.

Useful Commands

Using the “show ip protocols” command will help you determine if the router is configured as a normal/standard or stub or nssa area.

Default Route in OSPF

A default route is required to provide reach ability to external destinations. There are 2 methods to implement default routing in OSPF: either configuring the area as stub or using the “default-information originate” command on the ASBR, assuming a default route exists on the ASBR. This create creates a default route Type 5 LSA which is flooded in all OSPF areas (except stub areas).

Options to use with the “default-information originate” command:

  • “always” – always advertise the default route if no default route exists on ASBR.
  • metric” – sets the OSPF cost for the default route
  • metric-type” – defines the metric type (E1/E2) for the default route
  • route-map” – matches conditions in a route-map

     

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