Thursday, December 17, 2015

Limit a Summary / Redistributed Route to NSSA only (Stop 7/5 Translation)

Type 7 LSA is generated by an ASBR when it redistributes routes to a NSSA.
This LSA contains a "P" bit which tells other ABRs to translate the type 7 LSA into type 5 LSA when the LSA enters area 0.

If the P bit is set to 1 it is allowed for translation and if the P bit is 0 it says do not translate this LSA so that it will only remain in NSSA. This is useful when there are multiple ABRs in the NSSA while you need to traffic engineer the network. By the way, when the ASBR is the ABR it will automatically set this P bit to 0 for its originating type 7 LSAs to stop routing loops.

Configuration files can be downloaded from here.

Quick review of the topology:-
Area 1 is the NSSA, R2 is the ABR and R4 is the ASBR which redistributes its connected subnet of loopback 0 interface 192.168.4.1/32

1st let's look at the databases of R4 & R1





































So now you can see that the type 7 LSA which is advertised by the R4 (4.4.4.4) is translated to a type 5 LSA by the R2 (2.2.2.2)

Let's look at the LSA in detail



















It says "Type 7/5 translation" which actually means that the P bit is set to 1.
If you want to keep this route only inside NSSA just enter the following command in OSPF configuration of R4 which it is originating.

R4(config)#router ospf 1
R4(config-router)#summary-address 192.168.4.1 255.255.255.255 nssa-only



















Now you can see the translation is off. So it will not be learned by the backbone routers.




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