I’m using a NSSA on a number of Mikrotiks. I require the external routes to stay within the area and not be translated at the ABR. Is it possible on ROS6 / ROS7 to remove the p-bit of an OSPF type-7 (redistributed route) to stop translation to a type-5 when it reaches an ABR?
You cannot manipulate p-bit but on the area border router in input chain you can drop any unwanted route from NSSA
Ok, thanks. I’ve confirmed I am able to apply a route filter in that way.
I’m still not entirely sure of the Mikrotik route selection process of the ABR when connected to a NSSA. Specifically when the ABR receives an identical prefix as a type-5 (via backbone) and type-7 (via NSSA) LSA.
I have a NSSA with 2x ABR’s. One is also operating as a ASBR. The ASBR is distributing a type-5 into the backbone and a type-7 into the NSSA. I’ve confirmed that the P-bit is correctly not set on the type-7. When the type-5 and the type-7 arrive at the 2nd ABR, this router selects the path towards the NSSA next-hop to reach the advertised route, rather than the lower metric (directly connected) path via the backbone. I’ve tried replacing the 2nd ABR with a Cisco, which selects the path via the backbone as I would expect.
Can you confirm the Mikrotik expected route selection process in this scenario?
OSPF prefers routing within the area even if metrics are lower for the path over the backbone.
Yes. The external route is advertised in both areas (backbone 0.0.0.0 as a type-5 and area 0.0.0.6 as a type-7). The question is why does the Mikrotik ‘ABR2’ prefer the external route from ‘ABR1-ASBR’ via the NSSA rather than the backbone area route.
Now tested with a Juniper VMX too, which agrees with the Cisco, preferring the backbone route.