Which two statements are correct about tunnels? (Choose two.)

Tunnels operate by encapsulating original payload packets within a new header to traverse an intermediate network. Because of this encapsulation process, tunnels inherently add additional overhead to the packet size. For example, an IP-IP tunnel adds a 20-byte outer IPv4 header, which reduces the effective Maximum Transmission Unit (MTU) for the payload and often requires fragmentation management. Furthermore, for the encapsulated packet to successfully traverse the underlay network, the local tunnel endpoint must possess a valid routing table entry to reach the remote tunnel endpoint's IP address. Without this underlay route, the router cannot forward the encapsulated packets. Consequently, proper routing reachability between endpoints and accounting for encapsulation overhead are fundamental requirements for successful tunnel deployment and operation in Juniper networks.

A: BFD is fully supported and commonly used over tunnels (such as GRE or IP-IP) in Junos to provide rapid failure detection for routing protocols running across the tunnel.

C: IP-IP tunnels are stateless mechanisms. The router simply encapsulates and decapsulates packets based on static configuration without maintaining dynamic session states or tracking connection phases.

1. Juniper Networks TechLibrary. Junos OS Logical Interfaces User Guide. Section: "Understanding IP-IP Tunnels." This section details how IP-IP encapsulation adds a 20-byte outer IP header, explicitly confirming the addition of packet overhead (Option D).

2. Juniper Networks TechLibrary. Junos OS Routing Protocols User Guide. Section: "Configuring Unicast Tunnels." The prerequisites explicitly state that the routing table must contain a valid route to the tunnel destination address for the tunnel interface to function and forward traffic (Option B).

3. Juniper Networks TechLibrary. Junos OS High Availability User Guide. Section: "Understanding Bidirectional Forwarding Detection (BFD)." This document outlines the configuration and support for running BFD sessions over logical tunnel interfaces (gr- and ip- interfaces) to monitor path liveliness, invalidating Option A.