OSPF NSSA

Definition

To prevent a large number of external routes from consuming the bandwidth and storage resources of routers in an area, OSPF defines stub areas prohibited from importing external routes. However, stub areas cannot meet the requirements of the scenario that requires the import of external routes while preventing resources from being consumed by external resources. OSPF defines the NSSA to meet the requirements.

NSSAs are a new type of OSPF areas.

An NSSA is similar to a stub area in many ways. The difference between an NSSA and a stub area is that an NSSA can import AS external routes and advertise them to the entire OSPF AS, but do not learn external routes received from other areas on the OSPF network.

Figure 5-14 NSSA

N-bit

All routers in an area must be configured with the same area type. In OSPF, the N-bit, carried in a Hello packet, is used to identify the area type supported by a router. OSPF neighbor relationships cannot be established between routers configured with different area types.

Some manufacturers do not comply with the standard. They set the N-bit in both OSPF Hello and DD packets. To allow Huawei devices to interwork with these manufacturers' devices, set the N-bit in OSPF DD packets on Huawei devices.

Type 7 LSA

• Type 7 LSAs are a new type of LSAs that can be used only in NSSAs. Type 7 LSAs describe imported external routes.
• Type 7 LSAs are generated by an ASBR in an NSSA and flooded only in the NSSA where the ASBR resides.
• When the ABRs in the NSSA receive these Type 7 LSAs, they translate some of the Type 7 LSAs into Type 5 LSAs to advertise AS external routes to other areas on the OSPF network.

Translating Type 7 LSAs Into Type 5 LSAs

To advertise the external routes imported by an NSSA to other areas, Type 7 LSAs need to be translated into Type 5 LSAs.

• The Propagate bit (P-bit) in a Type 7 LSA indicates that whether the router needs to translate Type 7 LSAs into Type 5 LSAs.
• By default, the ABR with the largest router ID in an NSSA is selected for translating Type 7 LSAs into Type 5 LSAs.
• Only the Type 7 LSAs in which the P-bit is set to 1 and the FA is not 0 can be translated into Type 5 LSAs. The FA indicates that the packet to a specific destination address will be forwarded to the address specified by the FA.
• The P-bit in the Type 7 LSAs generated by ABRs is not set to 1.

Preventing Loops Caused by Default Routes

There may be multiple ABRs in an NSSA. To prevent routing loops, these ABRs do not calculate default routes advertised by each other.