Which two statements are true for GRE tunneling? (Choose two.)

Generic Routing Encapsulation (GRE) encapsulates payload packets within an outer IP packet for transport over an IP network. For this transport to succeed, the device originating the tunnel (the source endpoint) must have a valid route in its routing table to the IP address of the tunnel's destination endpoint. This ensures the outer IP packet can be forwarded correctly.

Within Juniper's Junos OS, tunnel interfaces such as GRE (gr-) are implemented as logical interfaces that have specific limitations. A key limitation is that they support the configuration of only a single logical unit, which must be unit 0.

B. GRE tunnels support multiple logical units per interface

This is incorrect. Junos OS explicitly restricts GRE (gr-) interfaces to a single logical unit (unit 0).

C. GRE tunnels are stateful by default

This is incorrect. GRE is a stateless tunneling protocol as defined in its RFC. It does not track connection states or sessions; it only provides an encapsulation mechanism.

1. Juniper Networks TechLibrary, Junos OS Interfaces Configuration Guide for Routing Devices, "Tunnel Services Interfaces Overview": This document states, "For gr-, ip-, pe-, and pd- interfaces, you can configure only one logical unit, unit 0." This directly supports answer D and refutes answer B.

2. Juniper Networks TechLibrary, Routing Policies, Firewall Filters, and Traffic Policers Feature Guide, "Example: Configuring a GRE Tunnel Between Two Routers": In the configuration steps, the guide specifies, "Configure a static route to the remote tunnel endpoint address..." This demonstrates the requirement for a valid route to the remote endpoint for the tunnel to function, supporting answer A.

3. IETF RFC 2784, "Generic Routing Encapsulation (GRE)", Section 1, Introduction: The RFC describes GRE as a mechanism for "encapsulation of an arbitrary protocol over another arbitrary protocol." The specification details a simple header format without any fields or procedures for state management, confirming its stateless nature and refuting answer C.