NSSA default-information-originate nssa-only keyword

The task solution shows the nssa-only keyword being used on R1 and R2 (the ABRs). The rationale in the solution guide is to clear the P-bit and prevent type 7 to type 5 translation.

When I configure this without the nssa-only keyword, it already appears the P-bit is cleared. I can't think of any reason that the nssa-only keyword is required here. Anyone have any thoughts about this?

R1#show ip ospf database nssa-external 0.0.0.0

OSPF Router with ID (192.122.3.1) (Process ID 100)

Type-7 AS External Link States (Area 121)

LS age: 982
Options: (No TOS-capability, No Type 7/5 translation, DC, Upward)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 192.122.3.1
LS Seq Number: 8000000D
Checksum: 0x3F27
Length: 36
Network Mask: /0
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 1
Forward Address: 0.0.0.0
External Route Tag: 0

LS age: 1871
Options: (No TOS-capability, No Type 7/5 translation, DC, Upward)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 192.122.3.2
LS Seq Number: 80000001
Checksum: 0x5120
Length: 36
Network Mask: /0
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 1
Forward Address: 0.0.0.0
External Route Tag: 0

Comments

  • Hi, 

    Same here, it works without nssa-only. I don't really understand what this keyword is for in the context of just defining the area. 

  • JoeMJoeM ✭✭✭

    I agree.   There is no reason for the keyword NSSA-ONLY.   The default route is only advertised into the NSSA area.

     

    I looked it up, and here is the CiscoDoc reference of nssa-only.   Now I am curious about when we would actually use this keyword, as it does not seem to be needed for this task.

    EDIT (testing):   I removed the default-info..originate from R2's nssa config. I was thinking that maybe without this, R2 might leak the default-information via E2.   Nope.  R2 does not even receive the default route via R16.

     

    http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/command/iro-cr-book/ospf-a1.html#wp2730600586

    nssa-only

    (Optional) Limits the default advertisement to this NSSA area by setting the propagate (P) bit in the type-7 LSA t zero.

  • From what I read, you can use the nssa-only on an ASBR that is not an ABR. I believe that would be used when you want to originate a default that is outside of OSPF to the NSSA and then not have that default propagated to the rest of the OSPF domain.

  • @ bengood24 - you are correct. The 'nssa-only' flag is not needed on R1 and R2.  

    The solution for this task has been updated to account for this. 

     

  • The solution was updated but not the solution explanation , which causes a confusion .

     

     

     

  • I don't see the wording of this question is clear , it is confusing and the solution explanation as well is not yet updated.

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