Question 93 - JN0-664 discussion

In the provided exhibit, PE-1 and PE-2 are configured with LDP-signaled pseudowires to provide Layer 2 connectivity between CE-1 and CE-2. The issue is that there is no connectivity between CE-1 and CE-2. Let's analyze the potential causes for this fault.

1. **LDP-Signaled Pseudowire (L2 Circuit) Configuration**:

- Pseudowires in MPLS networks use LDP (Label Distribution Protocol) to signal the virtual circuit (VC) labels between PE routers.

- For successful connectivity, the VC ID (Virtual Circuit Identifier) and LSPs (Label Switched Paths) between the PE routers must be correctly configured and operational.

2. **Analysis of the Exhibit**:

- The output shows the status of the L2 circuit connection on PE-1.

- The status (St) for the interface is `rmt Dn`, which indicates that the remote site (PE-2) is down or unreachable.

3. **Potential Causes**:

- **A. The VC IDs are mismatched**:

- Correct. If the VC IDs configured on PE-1 and PE-2 do not match, the L2 circuit cannot be established. Mismatched VC IDs prevent the pseudowire from forming correctly.

- **B. There is no LSP configured from PE-1 to PE-2**:

- Correct. LSPs are required for MPLS forwarding. If there is no LSP from PE-1 to PE-2, the LDP session cannot establish a path for the pseudowire. This results in the pseudowire being down.

- **C. Interface ge-0/0/0 on PE-1 is down**:

- Incorrect. The interface status is shown as `Up`, meaning the physical interface is operational.

- **D. There is no LSP configured from PE-2 to PE-1**:

- While this might seem like a potential issue, the specific problem of the remote site being down (`rmt Dn`) typically relates more directly to the forward path from PE-1 to PE-2 (i.e., no LSP from PE-1 to PE-2). Hence, the more accurate immediate cause is covered in Option B.

**Conclusion**:

Given the analysis, the correct statements describing potential causes for the fault are:

**A. The VC IDs are mismatched.**

**B. There is no LSP configured from PE-1 to PE-2.**

**Reference**:

- Juniper Networks Documentation on L2 Circuits: [Configuring Layer 2 Circuits](https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/layer-2-circuits-configuring.html)

- MPLS Configuration Guide: [Juniper MPLS Configuration](https://www.juniper.net/documentation/en_US/junos/topics/topic-map/mpls-overview.html)