Making Sense of OSPF Areas - some of the fundamentals of OSPF

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Making Sense of OSPF Areas - some of the fundamentals of OSPF

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• Relates to: CCNA

All of my customers run EIGRP. What that means is that the
networks that I work on only use Cisco routers, so I can run
Cisco's proprietary protocol. This means that I don't have
a clue when it comes to Open Shortest Path First (OSPF).
Not a good thing on the path to the CCIE. This week's
newletter outlines some of the fundamentals of OSPF.

OSPF is a link state routing protocol that updates the
routing table only when network changes occur, rather than
at a predefined interval. OSPF supports variable length
subnet masks (VLSM) and route summarization.

OSPF routers and links are grouped logically into areas that
are identified by assigned numbers. All OSPF networks have
at least one area with the default being area 0. If more
than one area exists, area 0 is defined as the backbone area
and is used to connect all other areas. Each area has its
own link state databases.

Stub areas are OSPF areas that have default routes, internal
and inter-area routes, but do not flood external routes.
A stub area has no routers or areas beyond it.

Totally Stubby Areas are stub areas that do not carry
inter-area routes (LSA types 3/4) as well.

A Not So Stubby Area is a hybrid area used to connect two
disparate organizations. External routes are advertised
using LSA type 7 that are less demanding on router
resources.

Different types of OSPF routers

Internal Router: Responsible for maintaining a current and
accurate database of subnets within the area. Forwards data
to other networks using the shortest path.

Backbone Router: Has an interface connected to the backbone
(Area 0).

Area Border Router (ABR): Has interfaces in multiple areas
with at lest one interface in area 0. Connects other areas
to the backbone and maintains routing information for each
connected area.

Autonomous System Boundary Router (ASBR): Router located
between OSPF autonomous system and a non-OSPF network.
Used to redistribute routing information between networks.
Must reside in a non-stub area.

OSPF LSA Types

Type 1: Router link advertisements generated by each router
for each area it belongs to. Flooded to a single area only.

Type 2: Network link advertisements generated by designated
routers describing the set of routers attached to a particular
network. Flooded to the area that contains the network.

Type 3/4: Summary link advertisements generated by ABRs
describing inter-area routes. Type 3 describes routes to
networks and is used for summarization. Type 4 describes
routes to the ASBR.

Type 5: Generated by the ASBR and describes links external
to the Autonomous System (AS). Flooded to all areas except
stub areas.

Type 6: Group membership link entry generated by multicast
OSPF routers.

Type 7: NSSA external routes generated by ASBR. Only
flooded to the NSSA. The ABR converts LSA type 7 into LSA
type 5 before flooding them into the backbone (area 0).

Realizing where each component of an OSPF network fits is
the first step to understanding how to effectively use OSPF.
Cisco provides lots of OSPF design information and advice.
Check out the following links for more information:

- Cisco's OSPF design guide
- Configuring OSPF (from the IP Routing Protocols manual)
- OSPF Commands (from Cisco IOS Command Reference Ver 12.2)