OSPF Type 7 to 5 Translation

• ABR with the highest router-id does the translation (avoid unnecessary and redundant LSA generation)
• Preserves forward address
• There is no type 4 ASBR summary (as forwarding address is preserved)
• Yet the calculation of the final forwarding path stays independent

OSPF Forwarding Address and How to Suppress It

If the forwarding address in an external LSA is specified, and this address is not reachable, the address contained in the LSA is not inserted into the route table. When NSSA ABR translates the type 7 NSSA LSA into they 5 LSA, by default the forwarding address is transferred from type 7 to type 5. The ABR can be configured to suppress the forwarding address during the translation, replacing the specified address with the address 0.0.0.0. When another router receives the type 5 external LSA with the forwarding address suppressed, instead of trying to direct traffic for the external address to the forwarding address the receiving router will attempt to direct the traffic to the toe 7 to type 5 translating ABR router.

area 10 nssa translate type 7 suppress-fa

Routing TCP/IP Volume 2, Second Edition

Which ABR Translates Type 7 to Type 5

In scenarios which there are more than one ABR connected to the NSSA area, only the ABR with the highest router-id may translate Type 7 LSA(s) to Type 5 LSA(s).

RFC 1587 - 3.4

http://gns3vault.com/OSPF/ospf-nssa-lsa-type-7-to-5-translator-election.html

OSPF P-bit

When external routing information is imported into an NSSA in a type 7 link-state advertisement (LSA), the type 7 LSA has only area flooding scope. To further distribute the external information, type 7 LSAs are translated into type 5 LSAs at the NSSA border. The P-bit in the type 7 LSA Options field indicates whether the type 7 LSA should be translated. Only those LSAs with the P-bit set are translated. When you redistribute information into the NSSA, the P-bit is automatically set. A possible workaround applies when the Autonomous System Boundary Router (ASBR) is also an Area Border Router (ABR). The NSSA ASBR can then summarise with the not-advertise keyword, which results in not advertising the translated type 7 LSAs.

http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a0080094704.shtml

NSSA and P-bit

If the NSSA's ABR receives a type 7 LSA with the P-bit set to one, it will translate the type 7 LSA into a type 5 LSA and flood it throughout the other areas. If the P-bit is set to zero, no translation will take place and the destination in the type 7 LSA will not be advertised outside of the NSSA.