OSPF NSSA P-bit Explained

Lessons Discussion
11

Pradeep,
I assume you mean the following command
translate type7 suppress-fa

To answer this question, you first need to understand how a Type-5 External advertisement changes when it is translated by an NSSA ABR. When an LSA Type-7 to Type-5 translation occurs (only an NSSA ABR can do this), two properties change: 1) The advertising router and 2) The Forwarding Address.

Regular Type 5 LSA
In a regular (not translated) Type-5 LSA, the Advertising router is the ASBR itself, and the Forwarding Address is 0.0.0.0

Regular Type 5 LSA

LS Type: AS External Link
  Link State ID: 172.16.16.0 (External Network Number )
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000001
  Checksum: 0x9985
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        MTID: 0 
        Metric: 20 
        Forward Address: 0.0.0.0

Type 7 to Type 5 Converted LSA
When an NSSA ABR converts from Type 7 to Type 5, it changes the Advertising Router to be the NSSA ABR that made the conversion, and it changes the Forwarding Address to be the ASBR (where the external route originated).

Here is the exact same scenario, but all that changes was the Area where the external route came from was converted to NSSA. Notice the change in the Advertising Router and the Forward Address

LS Type: AS External Link
  Link State ID: 172.16.16.0 (External Network Number )
  Advertising Router: 5.5.5.5
  LS Seq Number: 80000001
  Checksum: 0x1CD0
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        MTID: 0 
        Metric: 20 
        Forward Address: 10.1.25.2

Ok, let’s talk about why this change might matter. In order for OSPF to install a Type-5 route, the Forwarding Address must also be learned via OSPF. This is a key point. It isn’t good enough that an OSPF router knows how to get to the Forwarding Address–it must know how to get to it via OSPF.

Suppose, in the example above that a router is also running EIGRP, and it has a route to “10.1.25.2” via EIGRP. What would happen to the route for 172.16.16.0/24 assuming this is known only via OSPF? It wouldn’t be installed in the RIB! Why? Because the Forwarding Address would be installed in the RIB via EIGRP–since EIGRP has a better Administrative Distance.

This is an example of where “suppress-fa” might be useful. This command stops the NSSA ABR from making the change to the Forwarding Address. By changing the FA from 10.1.25.2 back to 0.0.0.0, OSPF is now allowed to install the 172.16.16.0/24 route into the RIB.

Below is the output for the same scenario, except this time I issued this command on the NSSA ABR
area 25 nssa translate type7 suppress-fa

LS Type: AS External Link
  Link State ID: 172.16.16.0 (External Network Number )
  Advertising Router: 5.5.5.5
  LS Seq Number: 80000002
  Checksum: 0x3DD4
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        MTID: 0 
        Metric: 20 
        Forward Address: 0.0.0.0

Notice that the Advertising Router remains at 5.5.5.5, but the FA has changed back to 0.0.0.0

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