Comprehensive and Detailed In-Depth Explanation:
1. Understanding MPLS LDP (Label Distribution Protocol)
* LDP (Label Distribution Protocol) runs over TCP port 646.
* LDP uses the transport address (Loopback0 in this case) to establish TCP sessions between neighbors.
* For an LDP session to be established, both routers must:
* Enable LDP on the interface (GE0/0/1).
* Allow TCP traffic on port 646 (used for LDP adjacency formation).
2. Analyzing the Issue and Possible Causes
* The figure shows a successful ping from R2 to R1's Loopback0 (10.0.1.1/32).
* This means basic IP connectivity between R1 and R2 is working.
* OSPF is properly advertising Loopback0 addresses between the routers.
* The problem must be related to LDP itself, not general IP reachability.
3. Evaluating Each Answer Option
* Option A: "R1's GE0/0/1 rejects packets with the destination IP address 10.0.2.2."
* Incorrect.
* The ping test in the figure shows successful replies from 10.0.1.1 to 10.0.2.2, meaning packets to 10.0.2.2 are not being rejected.
* This is NOT the cause of the LDP failure.
* Option B: "MPLS LDP is not enabled on R2's GE0/0/1."
* Correct.
* If LDP is not enabled on R2's GE0/0/1, then R1 and R2 cannot form an LDP session.
* MPLS LDP must be enabled on both interfaces for label exchange.
* This is a valid cause of the issue.
* Option C: "R2's GE0/0/1 rejects packets with TCP destination port 646."
* Correct.
* LDP operates over TCP port 646, so if R2's GE0/0/1 has a firewall or ACL blocking TCP port 646, then LDP will fail.
* Blocking TCP 646 on R2's interface would prevent the session from establishing.
* This is another valid cause of the issue.
* Option D: "R1's GE0/0/1 rejects packets with UDP destination port 646."
* Incorrect.
* LDP does not use UDP; it uses TCP port 646.
* This is not a valid cause of the issue.
Final Answer:
# B and C are correct.
HCIP-Datacom-Advanced Routing & Switching Technology References:
* MPLS LDP Session Establishment and TCP 646 Dependency
* How OSPF Advertises Loopback0 for LDP Transport
* Common LDP Debugging Techniques and ACL Issues