Understanding MPLS Label Retention Modes
In MPLS (Multiprotocol Label Switching), a Label Switching Router (LSR) can operate in one of two label retention modes:
1️⃣ Liberal Label Retention Mode (LLRM)
Stores all labels received from neighbors, even if they are not immediately used.
Advantage: Fast convergence during link failures.
Disadvantage: Consumes more memory & label space.
2️⃣ Conservative Label Retention Mode (CLRM)
Only keeps labels for the best next-hop path.
Advantage: Saves memory & label space.
Disadvantage: Slower convergence when topology changes.
Why are C and D Correct?
✅ C. An LSR reserves all labels distributed by its neighbor.
Correct: In Liberal Mode, the router accepts and stores all labels received, even for non-best paths.
✅ D. The liberal mode requires more memory and label space.
Correct: Since the router keeps all labels, it requires higher memory & storage.
Why are A and B Incorrect?
❌ A. An LSR retains labels from a neighboring LSR only when the neighbor is its next hop.
Incorrect: This describes Conservative Mode, not Liberal Mode.
❌ B. This label retention mode saves memory and label space.
Incorrect: Liberal Mode consumes more memory, not less.
Real-World Application
Liberal Mode is used in large-scale MPLS networks where fast failover is critical.
Conservative Mode is preferred in memory-constrained environments.
✅ Reference: Huawei HCIE-Datacom Study Guide – MPLS Label Retention Modes
