OSPF – NOTES

LSA Type 1:

• Router LSA
• Sent between routers in the same area, does not leave the area. Sends interface and adjacent neighbor information to all routers in this area.

LSA Type 2:

• Network LSA – Designated Router
• Floods its area in a broadcast network to advertise what routers are
  participating in OSPF, what the segment connections look like.

LSA Type 3:

• Summary LSA
• Packets are generated by ABR. The ABRA advertises summary prefixes across the area boundaries with the Type 3 LSA. Originates from the type 1 LSA, turns into type 3 at the area border.

LSA Type 4:

• ASBR Summary LSA
• Used to advertise that an ASBR is present to other areas in the Autonomous system. An ASBR sends out an LSA type 1, then the ABR changes to type 4 when injecting into other areas.

LSA Type 5:

• ASBR External LSA
• Used for route redistribution from either static or alternate routing protocols not part of the OSPF AS. The redistributed routes show up in internal AS route tables as E1 or E2.

LSA 7:

• NSSA External LSA
• Used in not so stubby areas to translate type 7 to type 5 throughout the network. the NSSA does not allow type 5, so ABR translates to type 5 and pushes throughout the rest of the network.

Why use more than one area?

• OSPF Autonomous systems that get very large are typically very resource intensive. The AS can be segmented into areas to suppress the amount of routes propagated across the entire OSPF domain. This means its less resource intensive for each router that participates in the AS.
• NOTE – Now sometimes used for security or organization. Resources in common network devices now have enough horsepower where areas are not needed quite as much.

Standard Area:

• A standard area uses LSA types 1, 2, 3, 4, and 5. All routers know the entire shortest path tree. They receive and pass along to other standard areas summary routes (LSA 3), ASBR notifications (LSA 4), and external/redistributed routes (LSA 5).

Stub Area:

• A stub area receives LSA types 1, 2, and 3. The routers in the stub area do not receive external routes (LSA 5) or the ASBR notification (LSA 4). The stub routers will receive the normal inter area types 1 and 2, then receive a summary LSA 3 for a default route. This allows for the route tables in the stub area to only have a default route for all external networks.

Totally Stubby Area:

• Totally stubby areas only receive default route injected from the ABR. The only LSA types that are available to the Totally stubby area are 1 and 2.

Not So Stubby Area:

• NSSA area can be used in conjunction with a Stub or Totally Stubby area. The NSSA allows an ASBR to advertise a route into the OSPF AS and NSSA area via a type 7 LSA. The ASBR receives the external route and sends the information via a type 7 LSA to the ABR. The ABR then translates the type 7 to type 5, which gets propagated to other areas in the network.