Refer to the exhibit. The company is running IS-IS in its core. A network engineer must segment the network into two areas to isolate departments. However, some networks must be shared from area 2…

Applying an ACL on RE to deny routes is typically used for packet filtering, not for directly controlling route advertisements in a routing protocol like IS-IS for inter-area filtering, and it lacks the specificity of route-tagging for inter-area sharing.

While route maps and distribute lists are used for filtering, implementing them inbound on RD from RE without a preceding mechanism like route-tagging makes it difficult to specifically identify and filter *shared* networks between the different areas based on the scenario's requirement.

Setting a unique area ID on each router would further segment the network but does not address the requirement of *sharing specific networks* between these isolated areas; it would actually increase isolation without a specific sharing mechanism.

Route-tagging allows specific routes to be identified with a unique tag as they are redistributed from one IS-IS area to another, typically at a Level 1-2 router. This tag can then be used in a route-map or distribute-list on a bordering router (like RD, a Level 2 router) to permit or deny their advertisement to other areas (e.g., from Area 2 to Area 1), ensuring only designated networks are shared.