Which tag is required when establishing a YANG-push subscription with a Cisco IOS XE device?

While `<yp:period>` is often included for periodic subscriptions, it is not strictly required for all YANG-push subscriptions (e.g., on-change subscriptions) and does not define the data itself.

`<yp:subscription-result>` is an element provided by the device in the *response* to an `establish-subscription` RPC, indicating the outcome, not part of the request.

`<yp:subscription-id>` is an identifier assigned by the device in the *response* to an `establish-subscription` RPC, enabling the subscriber to manage or terminate the specific subscription, not part of the request.

The `<yp:xpath-filter>` tag is essential and required when establishing a YANG-push subscription because it precisely defines the subset of data from the YANG data tree that the subscriber wants to receive, without which the device would not know what data to push.