Exam JN0-663 topic 1 question 92 discussion

The're are two options to leak external routes: 1 - Export policy on L1/L2 router (R2) or 2 - Enable wise metric on external router generator (R1). Because of that the only options that It seem correct is C.

The JUNOS software's default operation of IS-IS is to advertise both the small and wide metric TLVs in all LSPs. You can inform the local router to use only the wide metric TLVs with the wide-metrics-only command. The command is applied on a per-level basis and allows the local router to advertise larger metrics using only the extended TLVs. Note that wide metrics cannot differentiate between intenrnal and external prefixes. As a result, the use of only wide metrics results in the automatic leaking of external prefixes from level 1 areas into level 2 areas. You can use routing policy to block the leaking of external prefixes into the backbone area if needed.

B is correct for me. The explanation of funkeymonkey shows why we need the first part of option B "Turn on wide metrics on R1 and R2". Then we need "write an export policy on R4 from level 2 to level 1 matching the external routes" to send this specific external route to R5. In another case, R5 will have only a default route in its routing table because of the attached bit in the received R4 LSP

B is correct. By default, L1 external route is not advertised to L2. To advertise, use policy or wide metrics only is required.

A must be correct

A as L1 to L2 is always on, so for the routes to get to R5 you need a policy on R4 to leak L2 to L1

The use of only wide metrics results in the automatic leaking of external prefixes from L1 areas into L2 area, as the devices will no longer be able to differentiate between internal and external prefixes.