RFC3101 describes an OSPF area called not-so-stubby area (NSSA). The motivation behind NSSA is to allow OSPF Stub areas to carry External routes (routes learned using other routing protocols like RIP, EIGRP, BGP, etc). Stub areas are defined as areas that are not capable of importing routes external to OSPF.
N-bit
RFC 3101 defines a new bit N in the Options field and a new Type 7 LSA to support NSSA. Together, the N-bit (NSSA supported bit) and E-bit (External Routing Capability of the area) reflect an interface's external LSA flooding capability. When the peers exchange Hello messages, they check for the N-bit (should be set to 1) and E-bit (should be set to 0), along with Area ID. A mismatch in the Options field could result in failed adjacency.
Type 7 LSA
External routes are imported into OSPF NSSA as Type 7 LSAs by NSSA ASBR. The NSSA ASBR redistributes routes from different routing protocol(s) into OSPF, and vice versa. This router sets the E-bit in Router LSA flag. The ASBR originates a separate Type 7 LSA for each external route.
Type 7 LSAs are only flooded within the originating NSSA. The NSSA ABR (the routers connecting NSSA to backbone area 0) translates Type 7 LSA into Type 5 LSA, and flooded into the OSPF topology.
In Cisco IOS, by default, the NSSA ASBR always set the N/P-bit (Propagate) in the Options field of Type 7 LSA. The P-bit is not set only when the NSSA ASBR and NSSA ABR are the same router for the area. The P-bit tells the NSSA ABR to translate a Type 7 LSA to Type 5 LSA. These translated Type 5 LSAs copy the Forwarding Address (FA) from Type 7 LSA.
The following packet capture shows a Type 7 LSA
Type 7 Translator Election
If multiple NSSA ABR routers are present, it is recommended that not all ABRs perform Type 7-to-5 translation to avoid routing loops. RFC 3101 defines a configurable parameter NSSATranslatorRole which specifies whether a router will perform Type 7-to-5 translation or not. Cisco IOS does not support this parameter yet. Cisco IOS uses Router ID to elect the Type 7-to-5 translator.
If there exists multiple NSSA ABRs capable of performing Type 7-to-5 translation, the router advertising with higher Router ID is elected as the translator. The NSSA ABR that is no longer required to perform translation, flushes its Type 5 LSAs.
Sample Scenario
N-bit
RFC 3101 defines a new bit N in the Options field and a new Type 7 LSA to support NSSA. Together, the N-bit (NSSA supported bit) and E-bit (External Routing Capability of the area) reflect an interface's external LSA flooding capability. When the peers exchange Hello messages, they check for the N-bit (should be set to 1) and E-bit (should be set to 0), along with Area ID. A mismatch in the Options field could result in failed adjacency.
Type 7 LSA
External routes are imported into OSPF NSSA as Type 7 LSAs by NSSA ASBR. The NSSA ASBR redistributes routes from different routing protocol(s) into OSPF, and vice versa. This router sets the E-bit in Router LSA flag. The ASBR originates a separate Type 7 LSA for each external route.
Type 7 LSAs are only flooded within the originating NSSA. The NSSA ABR (the routers connecting NSSA to backbone area 0) translates Type 7 LSA into Type 5 LSA, and flooded into the OSPF topology.
In Cisco IOS, by default, the NSSA ASBR always set the N/P-bit (Propagate) in the Options field of Type 7 LSA. The P-bit is not set only when the NSSA ASBR and NSSA ABR are the same router for the area. The P-bit tells the NSSA ABR to translate a Type 7 LSA to Type 5 LSA. These translated Type 5 LSAs copy the Forwarding Address (FA) from Type 7 LSA.
The following packet capture shows a Type 7 LSA
Type 7 Translator Election
If multiple NSSA ABR routers are present, it is recommended that not all ABRs perform Type 7-to-5 translation to avoid routing loops. RFC 3101 defines a configurable parameter NSSATranslatorRole which specifies whether a router will perform Type 7-to-5 translation or not. Cisco IOS does not support this parameter yet. Cisco IOS uses Router ID to elect the Type 7-to-5 translator.
If there exists multiple NSSA ABRs capable of performing Type 7-to-5 translation, the router advertising with higher Router ID is elected as the translator. The NSSA ABR that is no longer required to perform translation, flushes its Type 5 LSAs.
Sample Scenario
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