This document discusses OSPF area types and router roles. It explains that an OSPF domain can be divided into areas with routers only aware of their area's topology. A backbone area connects all other areas and distributes routing information between them. Area Border Routers connect multiple areas, including the backbone. Autonomous System Border Routers redistribute external routes into OSPF. The document defines regular, stub, totally stub, and NSSA areas, explaining the LSAs each type permits and how routing information is distributed between areas.

1. OSPF AREA TYPES
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2. Area concept
• An OSPF domain can be divided into areas, which are
just a collections of Routers and Networks (links)
• Routers are only aware of the topology in its connected
areas
• In order to distribute the routing information between
areas, a special area is maintained – backbone area
• OSPF protocol maintains a strict hierarchy, where all
areas need to connect to the backbone area.

3. Router Roles
• Router connected to multiple areas is called Area Border
Router (ABR).
• Cisco definition of ABR requires a connection to the
backbone area
• A router redistributing external information into OSPF is
called Autonomous System Border Router (ASBR).

4. Distribution Between Areas
• Data (prefix reachability) distribution between areas is done
in a distance vector manner.
• Prefixes reachable from an area are advertised to other
connected areas in a form of inter-area announcements
• Loop prevention – only backbone area inter-area prefixes
are considered by ABR during the route calculation
• All intra-area and inter-area prefixes are advertised from
backbone area to non-backbone areas
• Only intra-area routes are advertised from
non-backbone area to backbone area

5. Area Types
• OSPF defines multiple area types, depending on what
LSAs are permitted to be flooded and generated inside
the area
• Regular area – all type of LSAs are flooded over it
• Backbone area is by definition a regular area
• Stub Area
• Totally Stub Area
• NSSA Area
• All routers in an area have to agree on an area type

6. Stub Area
• External LSAa (Type 5) are not flooded into the Stub
area
• External LSAs are not originated in Stub area
• ASBR cannot be placed inside the ASBR
• Virtual link cannot go through Stub area as this is part
of the backbone ( type 5 flooding capable)
• In order to assure reachability to external destinations
a default route ( LSA type 3 ) is advertised by the ABRs
into the Stub area.
• Possible sub-optimal path to external destinations

7. Totally Stub Area
• Inter area prefixes (Type 3 LSA) are not flooded to this
area
• routers within the area rely on default route for Inter Area
and External routing
• the price to pay is the sub-optimal routing outside of an
area
• Only ABRs in the Stub Area need to be configured as
Totally Stub as they have to inject type 3 LSA

8. NSSAArea
• NSSA area has the same restriction as Stub - no type 5 is
flooded into this area
• an ASBR can be placed inside this area
• The external information in NSSA area are flooded in
form of Type-7 LSAs.
• Type-7 LSAs have area flooding scope – never leave
area where they were generated
• To allow external information generated inside NSSA
area to be distributed outside of an area Type-7 LSAs
can be translated by the NSSA ABR to Type-5 LSAs,
which are flooded domain wise.

9. NSSAArea (cont.)
• To allow routers inside the NSSA area to reach external
destinations generated outside of an NSSA area, NSSA
ABR can be configured to generate a default route inside
NSSA area
• NSSA ABR generates Type-7 default route
• Required configuration
• nssa default-information-originate

10. Totally NSSAArea
• NSSA area where inter-area prefixes are not allowed,
similar to totally stub area
• Default route generated on NSSA ABR guarantees the
reachability for any destination outside of an area
• Configuration
•nssa no-summary

11. Location of Different LSAs
External
RIP
Network
ASB
R
DR
Area
11
ABR
ASBR
RIP Network
ABR
Area 0
Area 10ASBR
ABR Summary (type 3))
IRABRIR Router Link (type 1)
IRIR
External (type 5)
ASBR IRASBR Summary (type 4)
ABR  IR (about ASBR)
Network link
(type 2)
DR  IR
External (type 7)
ASBRIR
(only in NSSA)

12. Q&A