OSPF Area Types Deep Dive: Stub, Totally Stubby, and NSSA

Beyond Basic Multi-Area Design

The multi-area OSPF design covered earlier in this series already reduces each area's link-state database by keeping detailed Type 1 and Type 2 LSAs local to their own area, discussed earlier in this series regarding LSA types, while summarized Type 3 LSAs cross area boundaries. OSPF's specialized area types go further, specifically targeting Type 5 External LSAs -- routes learned from outside OSPF entirely via redistribution, discussed earlier in this series -- which can flood throughout the entire OSPF domain by default and significantly bloat routing tables in areas that have no actual need for that external routing detail.

Why External Routes Are Often Unnecessary in Leaf Areas

Consider a small branch office area connected to the rest of an enterprise network only through a single Area Border Router. Every router in that branch area needs to reach external destinations like the internet, but they do not need the full, detailed list of every specific external route -- a single default route pointing back toward the ABR accomplishes exactly the same practical outcome with a dramatically smaller routing table.

Stub Area: Blocking External Routes

A Stub Area prevents Type 5 External LSAs from entering the area entirely, with the ABR instead injecting a default route so area routers can still reach external destinations.

-- Configuration must match on EVERY router
-- within the stub area, including the ABR

ABR(config-router)# area 1 stub

Internal-Router(config-router)# area 1 stub

-- Every router must agree the area is a stub;
-- a mismatch prevents OSPF neighbor adjacencies,
-- discussed earlier in this series, from
-- forming correctly between routers that
-- disagree on this setting

Router# show ip route

O*IA 0.0.0.0/0 [110/2] via 192.168.1.1, GigabitEthernet0/0
O IA 192.168.2.0/24 [110/3] via 192.168.1.1, GigabitEthernet0/0

-- Notice: no O E2 (external) routes appear at
-- all -- only the injected default route and
-- normal inter-area routes remain

Totally Stubby Area: Blocking Inter-Area Routes Too

Cisco's proprietary Totally Stubby Area extends the stub area concept further, blocking not only Type 5 External LSAs but also Type 3 Summary LSAs from other areas -- leaving only a single default route and the area's own local, directly connected routes.

-- Only the ABR needs the "no-summary" keyword;
-- internal routers still just use "area X stub"

ABR(config-router)# area 1 stub no-summary

Internal-Router(config-router)# area 1 stub
-- (unchanged from the regular stub configuration)

Router# show ip route

O*IA 0.0.0.0/0 [110/2] via 192.168.1.1, GigabitEthernet0/0
C    192.168.10.0/24 is directly connected, GigabitEthernet0/1

-- Now even routes to OTHER areas within the
-- same OSPF domain are gone entirely -- replaced
-- by the single default route, resulting in
-- the smallest possible routing table for
-- this area

Since the totally stubby area configuration is Cisco-proprietary, it should only be used in networks running entirely Cisco equipment throughout that specific area -- a mixed-vendor environment would need to rely on standard stub areas instead.

The Fundamental Limitation Both Stub Types Share

Critical restriction: neither a stub area nor
a totally stubby area can contain an ASBR
(Autonomous System Boundary Router) -- the
router type responsible for redistributing
external routes into OSPF, discussed earlier
in this series

This makes sense structurally: since Type 5
External LSAs are entirely blocked from
entering these area types, an ASBR generating
those exact LSAs from within the area would
have nowhere for that information to actually go

NSSA: Stub Benefits with Local Redistribution Support

The restriction against containing an ASBR becomes a genuine problem for a common real-world scenario: a branch office area that still needs the reduced routing table benefits of a stub area, but also has its own local internet connection or other external route source requiring redistribution directly into that specific area. NSSA (Not-So-Stubby Area) solves exactly this scenario.

ABR-and-ASBR(config-router)# area 1 nssa

Internal-Router(config-router)# area 1 nssa

-- This area now behaves like a stub area
-- (blocking external routes originating
-- elsewhere in the OSPF domain) while still
-- permitting local redistribution to occur
-- from within this specific area

Type 7 LSAs: NSSA's Special Mechanism

NSSA solves this apparent contradiction using a new LSA type specifically invented for this purpose: routes redistributed within an NSSA are carried as Type 7 LSAs rather than the normal Type 5, remaining confined within the NSSA itself until the ABR converts them.

How NSSA route propagation actually works:

1. An ASBR within the NSSA redistributes an
   external route, generating a Type 7 LSA
   (rather than the normal Type 5)

2. Type 7 LSAs flood only within the NSSA itself,
   never crossing directly into other areas

3. The ABR translates each Type 7 LSA into a
   standard Type 5 External LSA before
   advertising it into the rest of the OSPF
   domain (typically into the backbone area)

This translation step at the ABR is the key mechanism making NSSA work: it allows external routes to originate from within an otherwise stub-like area while still keeping the area's own internal database free of the potentially large number of external routes that might exist elsewhere in the OSPF domain.

Verifying the Type 7 to Type 5 Translation

ABR# show ip ospf database nssa-external

            Type-7 AS External Link States (Area 1)

Router# show ip ospf database external

            Type-5 AS External Link States

-- The same underlying route appears as Type 7
-- within the NSSA, and as Type 5 once it has
-- been translated by the ABR and propagated
-- into the rest of the OSPF domain

Comparing the Three Specialized Area Types

Stub Area:
  Blocks: Type 5 (external routes)
  Allows: Type 3 (inter-area routes)
  Cannot contain an ASBR

Totally Stubby Area (Cisco-proprietary):
  Blocks: Type 5 AND Type 3
  Allows: only a single default route
  Cannot contain an ASBR

NSSA:
  Blocks: Type 5 from elsewhere in the domain
  Allows: Type 3, and locally-originated
    external routes via Type 7 (translated
    to Type 5 at the ABR)
  CAN contain an ASBR -- this is NSSA's
    defining advantage over a plain stub area

Why Choosing the Right Area Type Requires Understanding the Trade-Offs

Each of these specialized area types solves the same underlying problem -- reducing unnecessary routing information in areas that do not need it -- but with progressively different capabilities and restrictions. Correctly selecting between a standard area, a stub area, a totally stubby area, and an NSSA requires understanding not just the syntax of each configuration, but precisely which LSA types each design blocks or permits, and specifically whether that area needs to support local route redistribution through its own ASBR -- the deciding factor that makes NSSA necessary instead of a simpler, more restrictive stub area.