Ok another week another very interesting question from some students I'm mentoring in the Cisco360 program. This one relates to IGP lesson block 2 lab. Specifically task 1.3 which appears to have the goal of influencing which area border router (ABR) of a not-so-stubby area (NSSA) actually translates type 7 lsa's to type 5 lsa's. While the goal of the task is fairly straight forward, the examples in the answer key are less than ideal to put it nicely or to be more of straight forward the examples in the answer key are simply not correct.
The answer key has several problems as best I can tell.
First the external route is for prefix 126.96.36.199/24. By default this is a type 2 external so the metric will always be 20 and both R3 and R4 will advertise this prefix into area 30 with equal metrics. So other devices in area 30 should see both R3 and R4 as equal cost routes to that prefix and thus install both routes in the routing table. However, the answer key seems to suggest that R3 will be seen as having the best route to this prefix which is simply not true. Both R3 and R4 will have equal cost routes and so SW2 and SW1 should (and do when I lab it up) have both routes installed in their respective routing tables.
Second, the answer key seems to suggest that R3 will be the router that is doing type 7 to type 5 translation for 188.8.131.52. However, this is not the case. It will always be R1 that does this translation regardless if R3 has a higher router-id because RFC1587 allows for this:
"If a router is attached to another AS and is also an NSSA area border router, it may originate a both a type-5 and a type-7 LSA for the same network. The type-5 LSA will be flooded to the backbone (and all attached type-5 capable areas) and the type-7 will be flooded into the NSSA. If this is the case, the P-bit must be reset in the type-7 NSSA so the type-7 LSA isn't again translated into a type-5 LSA by another NSSA area border router. "
So R1 will originate a type 5 into the NSSA and a type 7 into the backbone and the type 5 will have the P-bit cleared so R3 will not also do the translation. Again, setting it up in the lab proves this to be the case:
SW2#show ip ospf data ext 184.108.40.206 | i Advertising Router
Advertising Router: 220.127.116.11
And both R1 and R3 have the P-bit cleared in the type 7 lsa so R3 will not also translate it:
R1#show ip ospf data nssa-ext 18.104.22.168 | i Options
Options: (No TOS-capability, No Type 7/5 translation, DC)
These are the types of commands I would expect to see for verification, but unfortunately only a 'show ip route' command is shown in the answer key. Regardless, I think the ultimate conclusion in the answer key is incorrect. Regardless of which device does the translation both paths (via R3 and R4) should be in the routing table of SW2 and R1 should be the translator.
If the lesson lab really wanted to show the difference in which router does the translation, the lab should be changed to have 22.214.171.124/24 be the external that gets advertised into the NSSA since R6 is not an ABR for the NSSA the ABR with the highest router ID will be the translator.