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The NSSA area has characteristics similar to the Stub area with one exception: the NSSA can have an external link (Figure 7d). Since LSA type 5 which is used to distribute routing information about the external links, are is prohibited in Stub areas, NSSA uses areas use LSA type 7 for this purpose. This LSA type has the same structure as LSA type 5, but it is permitted in NSSA areas. At the area border, the ABR converts the LSA type 7 to LSA type 5, setting itself as the routing source. Since the ABR performing the LSA conversion becomes the source, there is no need to generate an additional type 4 LSA.
Usually, the NSSA area usage is a result of the network's development: connecting an external communication channel to the Stub area requires changing its type to NSSA.
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One of the OSPF principles is to always connect two non-backbone areas only through the backbone area. Despite that, as a result of the historical development, the structure of some networks does not match with this principle. Bringing such networks up to the OSPF backbone are can be costly, so OSPF has been extended with the virtual link concept.
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- A virtual link is a logical connection configured on two ABRs, one of which is connected to the backbone area. Routers R1 and R2 are ABRs on which a virtual network interface is created and R2 is connected to the backbone area via the eth1 interface.
- The virtual link is the interface used by R2 to connect area 4. All LSA types are distributed over the virtual link like through a normal interface.
- The area that is common for two ABRs organizing sharing a virtual link is called a transit area. In the example below, area 7 is the transit area.
- The transit area should have the Normal type. It is not possible to establish a virtual link through Stub or NSSA areas.
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- Open implementation: OSPF is an open protocol, so it can be used by equipment from different manufacturers.
- Easy configuration: in small networks, the protocol can be started with only two commands.
- Flexible configuration: the wide protocol tools tool set allows to implement many network schemes.
- Scalability, fault tolerance, balancing, efficiency: similar to ODR, OSPF has the advantages of a dynamic routing protocol.
- High entry threshold: understanding the OSPF terminology and logic is time consuming.
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The examples on how to configure OSPF are present in the child page: OSPF protocol's configuration. |
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