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New Member

Totally NSSA and ASBR

Hi all,

Need to confirm few things below

Router R3 is ABR  and connected to R1.

R1 is ASBR as it is redistributing EIGRP into the OSPF.

This is what i did

R3

area 11 nssa no-summary -------------This makes it Totally NSSA right?

R1

area 11 nssa.

this is what  i saw as soon as i config R3 with

area 11 nssa no-summary  and did sh ip ospf

R3 shows ABR and ASBR ????????????????????????

So this means when we make ABR to totally NSSA  then it also becomes ASBR as it is redistributing LSA type 5 into OSPF domain????

Thanks

MAhesh

Everyone's tags (2)
2 ACCEPTED SOLUTIONS

Accepted Solutions
Hall of Fame Super Silver

Re: Totally NSSA and ASBR

Hello Mahesh,

yes with OSPF NSSA area the ABR between (0, NSSA-area) has the duty to convert LSA type 7 generated by the ASBR internal to NSSA area (R1 in your case)  to OSPF LSA type5 and in this it becomes an ASBR.

Conversion is performed if the propagation bit P in the LSA type 7 allows for it. This gives you the capability to generate LSA type 7 that are not translated at area border.

Note that this happens also for NSSA area not only for totally NSSA

Hope to help

Giuseppe

Hall of Fame Super Silver

Totally NSSA and ASBR

Hello Vijay,

very good observation you are developing a taste for details!

the P-bit is the NP bit in your capture the meaning changes when the options are within an LSA type 7.

see

http://www.ietf.org/rfc/rfc3101.txt

Appendix A: The Options Field

the meaning is N bit = NSSA area when the options field is in an hello packet, it is a P bit in the options field of an LSA type 7.

N-bit:  The N-bit describes the router's NSSA capability.  The N-
              bit is used only in Hello packets and ensures that all
              members of an NSSA agree on that area's configuration.
              When the N-bit is set in the Hello packet that is sent out
              a particular interface, it means that the router will send
              and receive Type-7 LSAs on that interface.  Two routers
              will not form an adjacency unless they agree on the state
              of the N-bit.  If the N-bit is set in the options field,
              the E-bit must be clear.

      P-bit:  The P-bit is used only in the Type-7 LSA header.  It flags
              the NSSA border router to translate the Type-7 LSA into a
              Type-5 LSA.  The default setting for the P-bit is clear.

if the P bit is set translation occurs at ABR at area border.

note: if two ABRs  for the NSSA area exist only one (the one with highest OSPF RID , the common choice ) performs the translation but both generate LSA type 3 for the forwarding address

Hope to help

Giuseppe

5 REPLIES
Hall of Fame Super Silver

Re: Totally NSSA and ASBR

Hello Mahesh,

yes with OSPF NSSA area the ABR between (0, NSSA-area) has the duty to convert LSA type 7 generated by the ASBR internal to NSSA area (R1 in your case)  to OSPF LSA type5 and in this it becomes an ASBR.

Conversion is performed if the propagation bit P in the LSA type 7 allows for it. This gives you the capability to generate LSA type 7 that are not translated at area border.

Note that this happens also for NSSA area not only for totally NSSA

Hope to help

Giuseppe

New Member

Re: Totally NSSA and ASBR

Hi Giuseppe..

Just got curious about this thread.

I'm just trying to understand this statement

"Conversion is performed if the propagation bit P in the LSA type 7 allows for it."

I did a packet capture to find out which bit were you talking about but just could not find any relavant fields. Am i looking at the wrong place ?

Hall of Fame Super Silver

Totally NSSA and ASBR

Hello Vijay,

very good observation you are developing a taste for details!

the P-bit is the NP bit in your capture the meaning changes when the options are within an LSA type 7.

see

http://www.ietf.org/rfc/rfc3101.txt

Appendix A: The Options Field

the meaning is N bit = NSSA area when the options field is in an hello packet, it is a P bit in the options field of an LSA type 7.

N-bit:  The N-bit describes the router's NSSA capability.  The N-
              bit is used only in Hello packets and ensures that all
              members of an NSSA agree on that area's configuration.
              When the N-bit is set in the Hello packet that is sent out
              a particular interface, it means that the router will send
              and receive Type-7 LSAs on that interface.  Two routers
              will not form an adjacency unless they agree on the state
              of the N-bit.  If the N-bit is set in the options field,
              the E-bit must be clear.

      P-bit:  The P-bit is used only in the Type-7 LSA header.  It flags
              the NSSA border router to translate the Type-7 LSA into a
              Type-5 LSA.  The default setting for the P-bit is clear.

if the P bit is set translation occurs at ABR at area border.

note: if two ABRs  for the NSSA area exist only one (the one with highest OSPF RID , the common choice ) performs the translation but both generate LSA type 3 for the forwarding address

Hope to help

Giuseppe

New Member

Totally NSSA and ASBR

Hi Giuseppe,

Many thanks again.

Need to learn lot from you guys.

Regards

MAhesh

New Member

Totally NSSA and ASBR

Thanks Giuseppe,

I learned something new today.. Thanks again.

-Vijay

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