OSPF Area Types and LSA's

Link State Advertisement (LSA) Types have never been my strong suite. I made a visual representation of how they are forwarded to help me get a better grasp on them.

• Default - This isn’t actually an LSA type. It’s meant to represent a default route.
• Type 1 - Router Link - This is generated by a router and lists the neighboring routers and the cost of each router. This is only flooded within the area that the router interface exists in.
• Type 2 - Network Link - This is generated by the Designated Router (DR). This lists all routers within an adjacent segment. This is only flooded within the area that the router interface exists in.
• Type 3 - Network Summary - This is generated by an Area Border Router (ABR) and is advertised among all areas, except totally stubby areas and totally not-so-stubby areas.
• Type 4 - ASBR Summary - This is generated by an ABR informing other areas that an Autonomous System Border Router (ASBR) exists within it’s network.
• Type 5 - External Link - This is generated by an ASBR and is flooded within its area. ABRs will also flood their connected areas with this type 5 LSA informing other areas that external networks exist within its area.
• Type 7 - NSSA External Link - This is generated by an ASBR participating in an not-so-stubby (NSSA) area. When the type 7 LSA reaches the ABR, It is converted to a type 5 LSA and forwarded to other areas informing them that an external network exists within its area.

When a type 5 LSA enters a stubby area, it’s entered into the RIB as a default route, striping the type 5 LSA routes. Type 5 LSAs are also not forwarded to not-so-stubby areas. To get external routes INTO an not-so-stubby area, the area has to be converted to a totally not-so-stubby-area, which will remove all summaries and inject a default route.

As you can see from the diagram, above, there are six area types.

• Backbone Area
• Standard Area
• Not-so-stubby Area
• Totally stubby Area
• Totally not-so-stubby Area

All areas have to be directly connected to area 0, which is the backbone area. All routers that exist within only area 0 or within just a single area are called backbone routers. Routers that connect to multiple OSPF areas are called ABRs. Routers that are connected to an OSPF area AND connected to an external routing protocol (EIGRP, ISIS, RIP, BGP, static routes, etc) are called ASBRS.

Stubby areas and totally stubby areas are areas that have one way in and one way out of an area. That is, they can’t be connected to area 0 and any other area or external network at the same time, as a stubby and totally stubby area ABR will not forward type 5 LSAs to other areas. A totally stubby area will only have a default route injected into its network.

Not-so-stubby and totally not-so-stubby areas can be connected to external networks and will forward the type 7 LSAs beyond their area. When the type 7 LSAs reach an ABR, the ABR will convert them to type 5 and forward them to the other areas.