OSPF - summary-address nssa-only on an ABR

When implementing OSPF using a NSSA it is possible to prevent an ABR from translating Type 7 LSAs to Type 5 LSAs by setting the P-bit. This is achieved by configuring the ASBR appropriately.

However, it is also possible to achieve something similar by using the summary-address nssa-only command on the ABR of the particular NSSA.

When applied on the ABR, this command will act somewhat similarly to the area nssa no-redistribution command. Even if Type 7 LSAs that are received from the NSSA's ASBR have the P-bit set, the ABR will not translate them into Type 5 LSAs. Essentially, the ABR is being instructed to ignore the P-bit in this situation and not perform the 7/5 translation.

Remember the summary-address nssa-only command primarily serves to control route summarization. However, when applied to an ABR, it has this side-effect of also influencing the translation of Type 7 to Type 5 LSAs.

If your goal is to purely control LSA translation, it is preferrable to use commands specifically designed for that purpose (like area nssa no-redistribution). This helps make the configuration intent more explicit and understandable to others who might review or maintain the configuration in the future.

Indeed this is undocumented behavior since there is no description of how this feature reacts when implemented on an ABR (at least I was unable to find any). However, it is not completely unexpected, since any manipulation of the P-bit will cause the ABR to obey that indicator.





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