PPPoE server can create a tunnel to a client if there is a router between them.

A PPPoE (Point-to-Point Protocol over Ethernet) server establishes a direct connection with a client device by creating a tunnel for data transmission. This protocol is typically established directly between a client and server, requiring them to be on the same local network segment or have a direct link. If there is a router situated between the PPPoE server and the client, the encapsulated PPPoE frames might not be properly forwarded, leading to connectivity issues.

Routing functionality can prevent proper tunneling because the router may not know how to handle the specific PPPoE frames. For a PPPoE connection to work effectively, both the server and client need to be able to communicate directly without any intermediary devices that manipulate or alter the PPPoE frames. Thus, for a PPPoE server to establish a tunnel to a client, they must be directly connected or when a router is involved that can handle PPPoE traffic appropriately. In typical scenarios, it is understood that there shouldn't be a router acting as an obstacle.