Definition

As defined in OSPF, stub areas cannot import external routes. This prevents a large number of external routes from consuming bandwidth and storage resources of the Routers in stub areas. To import external routes and to prevent external routes from consuming resources, NSSAs are used, because stub areas cannot meet requirements.

NSSAs are a new type of OSPF areas.

There are many similarities between NSSAs and stub areas. The difference between NSSAs and stub areas is that NSSAs can import AS external routes into the entire OSPF AS and advertise the imported routes in the OSPF AS, but do not learn external routes 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 is carried in a Hello packet and is used to identify the area type supported by the Router. OSPF neighbor relationships cannot be established between Routers configured with different area types.

Some manufacturers do not comply with the standard and 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 only be used in NSSAs and describe the imported external routes.
• Type 7 LSAs are generated by ASBRs in an NSSA and flooded only in the NSSA where the ASBRs reside.
• 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 the 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 so that the external routes can be advertised on the entire OSPF network.

• The Propagate bit (P-bit) in a Type 7 LSA is used to instruct the Router whether to translate Type 7 LSAs into Type 5 LSAs.
• By default, the ABR with the largest router ID in an NSSA is responsible 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 is not set for default routes in Type 7 LSAs generated by an ABR.

Preventing Loops Caused by Default Routes

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