Type 7 to Type 5 LSA Translation in OSPF

[rocky2024]

Hello,

as per books, its written that

IF THE NSSA'S ABR RECEIVE A TYPE 7 LSA WITH THE P BIT SET TO 1, THE TYPE 7 LSA TRANSLATES INTO A TYPE 5 LSA BEFORE BEING FLOODED TO OTHER AREAS

my question is how and when P Bit set to 1 ? and who will set to 1 ? and why ?

please help to understand this point

\thank you

[Shambhu-Kumar]

Hello Rocky1

Objective of having an NSSA is to allow external routes (learned from different routing domains) into that area, and they will be carried as Type-7 LSA.

P-bit = (P - propagate) only used in type-7 LSAs to tell the ABRs to translate that type-7 LSA into a type-5 LSA and found in the options field of an LSA type 7.

N-bit = used in the Hello packets to indicated that the router has NSSA capability on that interface (two routers will not form an ajancency unless they agree on the N-bit, meaning they are both configured as NSSA for that interface)

The ASBR (connected to other routing domains) will send the external prefixes as type-7 LSAs with the P-bit set. These LSAs will be translated into type-5 LSAs by ABRs.
When we redistribute prefixes into an OSPF NSSA area then these prefixes are flooded within the NSSA area as LSA type 7. Once these prefix crosses ABR, they are translated into LSA type 5 and advertised to other areas.
The P bit instructs ABR, convert LSA type 7 into LSA type 5. This bit will be set for all the prefixes that are redistributed.

[rocky2024]

Hello Shambhu,

thank you for replying.

but still i want to understand when asbr will set p bit to 1 or 0 ?

when asbr and how asbr will decide to set p bit 1 or 0 ?  0 means dont convert from lsa 7 to 5 right ? and p bit set to 1 means convert lsa 7 to 5 so when asbr will decide whether to set 1 or 0 ?

[Shambhu-Kumar]

Hello Rocky1

In Cisco IOS, by default, the NSSA ASBR always set the P-bit in the Options field of Type 7 LSA.This bit will be set for all the prefixes that are redistributed.
But P-bit is not set only when the NSSA ASBR and NSSA ABR are the same router for the area. If bit P = 0, then the NSSA ABR must not translate this LSA into Type 5

Also The P-bit is set to zero when using

summary-address 

command with option

nssa-only

ABR does NOT translate LSA type 7 to LSA type 5 when receiving P-bit zero. Therefore, other areas which includes backbone do NOT learn external routes.

[rocky2024]

Hello Shambhu,

But P-bit is not set only when the NSSA ASBR and NSSA ABR are the same router for the area. If bit P = 0, then the NSSA ABR must not translate this LSA into Type 5 -----------so how external prefixes are being advertised to rest network from same NSSA ABR/ASBR ? why its set to 0 ?  are they being advertised as type 7 only to rest network if p bit set to 0 ?

[Shambhu-Kumar]

There are situations where there is no need to inject external routes into the NSSA as type 7. This situation usually occurs when an ASBR is also an NSSA ABR. When redistribution takes place in this scenario, the router generates type 5 as well as type 7 LSAs. You setup the router so that it does not create type 7 LSAs for NSSA with this command:

Router(config)#router ospf 1
Router(config-router)#area 1 nssa no-redistribution

In the diagram, Area 1 is configured with the no-redistribution option. This means that all EIGRP routes are redistributed into area 0, but no type 7 LSAs are generated for Area 1. Configure this command only on an NSSA ASBR that is also an ABR.

[MHM Cisco World · Accepted solution]

this simple lab, 
in R1-R2 the ospf area is 1 and it NSSA, 
in R2 we have static route toward R3 
how P bit set 
1- P  it is default 1 in LSA7)
2- P be 0 when I use

redistribute static subnet nssa-only

 

what P =1 meaning? meaning any ABR of this NSSA area can translate this LSA7 into LSA5 
what P=0 meaning? meaning any ABR of this NSSA area CAN NOT translate this LSA7 into LSA5

where we use it ?
if we want to isolate the external prefix learn from NSSA from other OSPF area we use this bit.