By Default, Advertised Address(es) in Junos OS

C

LDP (Label Distribution Protocol) is a protocol used to establish and maintain Label Switched Paths (LSPs) in MPLS (Multiprotocol Label Switching) networks. An LDP Forwarding Equivalence Class (FEC) is a group of IP addresses that are assigned the same label by the LDP. When a packet with a certain IP address arrives at a router, the router looks up the label assigned to the FEC and forwards the packet based on that label.

Now, coming to the question, by default, the Junos OS advertises the loopback address in LDP Forwarding Equivalence Class (FEC) messages. The loopback address is a logical IP address assigned to a router's loopback interface. The loopback interface is always up and running, and it is used for management, testing, and troubleshooting purposes.

When a router advertises its loopback address in LDP FEC messages, it allows other routers in the network to use that address as the next-hop for forwarding packets. This provides a stable and predictable routing path in the network, even if physical interfaces go up and down.

Option A (all router addresses) and Option B (all transit interfaces) are incorrect because these options do not provide a clear indication of which IP address should be used as the next-hop for forwarding packets.

Option D (primary transit interface address) is also incorrect because it only provides the IP address of the primary transit interface, which may not always be available or stable.

Therefore, the correct answer is C (loopback address), which is the default address advertised by the Junos OS in LDP Forwarding Equivalence Class (FEC) messages.