While troubleshooting an AToM L2VPN service, a network consultant notices that the AC Layer 2 encapsulations are different. Which action should the consultant take in order to make the MPLS L2VPN…

Question

Options

• Atag-rewrite on the ingress and egress PE router
• Binterworking IP configuration on the last PE router before label disposition
• Cnonrouted interworking setup to properly translate only the Layer 2 information from the AC
• Dinterworking IP configuration on both the AC terminations on the PEs

• A
  4% (2)
• B
  15% (8)
• C
  8% (4)
• D
  73% (38)

Explanation

When AToM pseudowires connect ACs with mismatched Layer 2 encapsulations (e.g., Ethernet on one PE, Frame Relay on the other), IP interworking must be configured on both PE routers where those ACs terminate. Each PE must independently strip its local Layer 2 header, pass the IP payload across the MPLS pseudowire, and re-encapsulate it in the peer's Layer 2 format - a bilateral operation that cannot function if only one side is configured.

Why the distractors fail:

• A (tag-rewrite): Tag rewriting manipulates MPLS labels, not Layer 2 encapsulations. It solves label-plane mismatches, not AC protocol mismatches.
• B (interworking IP on last PE only): A one-sided configuration leaves the ingress PE unable to properly signal or handle the encapsulation translation; both endpoints must participate.
• C (nonrouted/Ethernet interworking): Nonrouted interworking is used when both ACs share the same Layer 2 protocol family and only minor header differences exist. When protocols differ fundamentally, you must go up to Layer 3 (IP), which requires IP interworking - not nonrouted interworking.

Memory tip: Think of IP interworking like a phone interpreter - you need one at both ends of the call. If only one PE "speaks the translation," the other end still can't understand the foreign encapsulation. "Both ACs, both PEs" is the rule.

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