An engineer is configuring a GRE tunnel interface in the default mode. The engineer has assigned an IPv4 address on the tunnel and sourced the tunnel from an Ethernet interface. Which option also is r

A Generic Routing Encapsulation (GRE) tunnel requires both a source and a destination IP address to be explicitly configured on the tunnel interface. The `tunnel destination <ip address>` command specifies the IP address of the remote tunnel endpoint, which is essential for the router to know where to encapsulate and send the GRE packets.

The `keepalive` command is optional and used for monitoring tunnel interface status, but it's not strictly required for the tunnel to become operational.

The `ip mtu` command adjusts the Maximum Transmission Unit for the tunnel, which can prevent fragmentation issues but is not a fundamental requirement for the tunnel's basic operational state.

The `ip tcp adjust-mss` command is used to prevent TCP reassembly issues by modifying the TCP Maximum Segment Size, but it is not essential for the GRE tunnel itself to be operational.