Tuesday, 27 September 2016

OSPF - Part 8

OSPF Area Types

Large OSPF domain is typically broken into separate to restrict the propagation of route and the amount of resources required by each router to maintain its link state database.

OSPF have several types of LSA to communicate link state information between neighbors.

OSPF Area types:

1.) Backbone area
        That is area 0, it require to connectivity between two different single areas.

2.) Standard area

In the example above, router 2 acts as the area border router (ABR) between a standard area and the backbone area. R3 is redistributing routes from an external domain, and is therefore designated as an autonomous system boundary router (ASBR).

As mentioned, type 1 and 2 LSAs are being flooded between routers sharing a common area. This applies to all area types, as these LSAs are used to build an area's shortest-path tree, and consequently only relevant to a single area. Type 3 and 5 LSAs, which describe internal and external IP routes, respectively, are flooded throughout the backbone and all standard areas. External routes are generated by an ASBR, while internal routes can be generated by any OSPF router.

Note the peculiar case of type 4 LSAs. These LSAs are injected into the backbone by the ABR of an area which contains an ASBR. This is to ensure all other routers in the OSPF domain can reach the ASBR.

Standard areas work fine and ensure optimal routing since all routers know about all routes. However, there are often situations when an area has limited access to the rest of the network, and maintaining a full link state database is unnecessary. Additionally, an area may contain low-end routers incapable of maintaining a full database for a large OSPF network. Such areas can be configured to block certain LSA types and become lightweight stub areas.

3.) Stub area 

In this example, r2 and r3 share a common stub area. Instead of propagating external routes (type 5 lsa)into the area, the ABR injects a type 3 LSA containing a default route into stub area. This ensure that routers in the stub area will be able to route traffic to external destinations without having to maintain all of the individual external routes. Because external routes are not received by the stub area, ABRs also do not forward type 4 LSAs from other areas into the stub. 


 Router(config-router)# area 2 stub

4.) Totally  Stubby area

Like stub areas, totally stubby areas do not receive type 4 or type 5 LSAs from their ABRs.  However, they also do not receive type 3 LSAs. All routing out of the area relies on the single default route injected by the ABR.

A stub area is extended to a totally stubby area by configuring all of its ABRs with the no-summary parameter.

Router(config-router) # area 2 stub no-summary

Stub and totally stubby area can certainly by convenient to reduce the resource utilization of routers in portions of the network not requiring full routing knowledge. However, neither type can contain an ASBR, as type 4 and 5 LSAs are not permitted inside the area. To solve this problem and in what is arguably the worst naming decision ever made, cisco introduced the concept of a not-so-stubby Area (NSSA).

5.)  Not-So-Stubby-area (NSSA)

An NSSA makes use of type 7 LSAs, which are essentially type 5 LSAs in disguise. This allows an ASBR to advertise external links to an ABR, which converts the type 7 LSAs into type 5 before flooding them to the rest of the OSPF domain.
An NSSA can function as either a stub or totally stubby area. To designate a normal (stub) NSSA, all routers in the area must be so configured

Router(config-router)# area 2 nssa

Type 3 LSAs will pass into and out of the area. Unlike a normal stub area, the ABR will not inject a default route into an NSSA unless explicitly configured to do so. As traffic cannot be routed to external destinations without a default route, you'll probably want to include one by appending default-information-originate (thanks to Adam for pointing this out)

Router(config-router)# area 2 nssa default-information-originate

To expand an NSSA to function as a totally stubby area, eliminating type 3 LSAs, all of its ABRs must be configured with the no-summary parameter:

Router(config-router)# area 2 nssa no-summary

The ABR of a totally stubby NSSA (or not-so-stubby-area, if you prefer) injects a default route without any further configuration. 

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