It is hard to understand the concepts, configuration and troubleshooting of normal OSPF areas. This article extends on these topics and covers the specifics inside the OSPF stub area types. The article on normal OSPF area troubleshooting reviews the different types of OSPF areas that can be used; this article will take a look at the different Link State Advertisement (LSA) types as well as a deeper look at what the stubby area types provide and how they are used to control the different LSAs that are allowed within each type of stubby area.
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What are LSAs?
The reachability information that is passed between the different routers in an OSPF domain are communicated via LSAs. There are several different types of LSA that communicate different pieces of information. In a normal area, all LSA types are allowed and communicated between the devices. A list of the different common LSAs is shown below:
- LSA Type 1 – Router Link Advertisements – All OSPF devices send this LSA type, it contains the states of all of the interfaces that the device has in the OSPF domain. This LSA type is kept within a single area.
- LSA Type 2 – Network Link Advertisements – Only Designated Routers (DR) send this type of LSA, it contains a list of devices connected to a particular network. This LSA type is kept within a single area.
- LSA Type 3 – Summary Link Advertisements – Only Area Border Routers (ABR) send this type of LSA, it contains a list of inter-area routes and network reachability information. This LSA type is sent into the different areas that are connected to the ABR (i.e. routes are exchanged between connected areas).
- LSA Type 4 – Summary Link Advertisements – Only Area Border Routers (ABR) send this type of LSA, it contains a list of inter-area routes to the Autonomous System Boundary Routers (ASBR) within the OSPF domain. This LSA type is sent into the different areas that are connected to the ABR (i.e. Routes to the different ASBRs in the OSPF domain are exchanged)
- LSA Type 5 – Autonomous System (AS) external link advertisements – Only ASBRs send this type of LSA, it contains a list of external routes that are reachable via the ASBR. This LSA type is flooded throughout the entire OSPF domain.
- LSA Type 7 – Not So Stubby Area advertisements – Only NSSA ASBRs send this type of LSA, it contains a list of external routes that are reachable via the NSSA ASBR. This LSA type differs from LSA type 5 because stubby areas do not allow LSA type 5, the workaround is to use LSA type 7 within the NSSA area which is then converted by the NSSA ABR into LSA type 5 which is then flooded into the rest of the OSPF domain.
Stubby Area Review
For those readers who have not read the previous OSPF area troubleshooting article, this section reviews the different types of OSPF stubby area and provides high level review of what they allow and deny into the specific area.
- Stubby – A stubby area is an area that is not able to accept LSA type 5 which contains routes from outside the OSPF domain (Redistributed into OSPF), these are also referred to as external routes. Traffic that is destined for a location outside of the OSPF network is only able to exit the area via a default route (which is injected by the ABR).
- Totally stubby – A totally stubby area is an area that is not able to accept LSA type 3 or 5 which contain routes from outside the OSPF domain and is not able to accept routes from outside the area; traffic is only able to exit the area via a default route (which is injected via the ABR).
- Not-so-stubby – A not-so-stubby area (NSSA) is an area that acts the same as a stubby area with one exception, while a stubby area is not able to accept LSA type 5 and is not able to connect directly to an external routing domain; an NSSA network is able to contain an ASBR (and connect to an external routing domain) by using an LSA type 7. The routes to this external routing domain are then injected into the backbone (and from there to other areas) by the NSSA ABR. This area is still not able to accept an LSA type 5 from the rest of the OSPF domain.
- Totally NSSA – A totally NSSA is an area that acts the same as a totally stubby area but with the same abilities as a normal NSSA area. As with a normal totally stubby area, a totally NSSA is not able to accept LSA 5’s (external) from other areas.
Stubby Area Troubleshooting
Without a proper understanding of what stubby areas provide, it is easy to get into trouble; this is why this article began with a review of LSA types and how the different stubby area types deal with the different LSA types. The most common issues that are seen are related to breaking the basic rules behind how to configure stubby areas, these include:
- Stubby areas are not able to contain ASBRs as they are unable to accept or transmit LSA type 5 which are required for an ASBR to work correctly. If an ASBR must be placed in a stub area, a not-so-stubby area must be used as it provides a workaround by using LSA type 7 inside the stub area.
- Stubby areas cannot be used to transport virtual links
- Area 0 (Backbone) cannot be a stubby area
- All OSPF devices inside the area must be configured as stub or NSSA devices, configuration of totally stub or totally NSSA areas is required on the connecting ABR.
Follow these basic rules and the troubleshooting related to the stub configuration of the network will be minimized.
As the name suggests, stubby areas are intended to be stub (or single entry) areas, as long as their purpose is restricted to this function they will provide trouble free operation.
The ability to restrict the types of OSPF packet types that are able to be sent into areas is a very important tool when dealing with lower power routing devices. OSPF networks are able to be used in very large environments and having the entirety of this routing information be processed and injected into a low power branch style router is just asking for trouble. Keep in mind what the purpose of the stubby areas is and they can be used to greatly improve the performance of the OSPF network. Hopefully the contents of this article will enable a better understanding of OSPF stubby areas and enable less troubling OSPF implementation.