In MPLS LDP (Label Distribution Protocol), label retention modes control how labels are stored in the Label Forwarding Information Base (LFIB).
DU Label Advertisement Mode (Downstream Unsolicited):
* Labels are advertised to all LDP peers without a request.
* The receiving router decides which label to use for forwarding.
Liberal vs. Conservative Label Retention:
# Liberal Label Retention Mode (Correct Answer)
* All received labels from all LDP peers are stored.
* Even if a peer is not the optimal next hop, its label is retained.
* Allows faster convergence during failures.
# Conservative Label Retention Mode
* Only the label from the best next-hop peer is stored.
* Reduces memory usage but can slow down convergence in case of failure.
Reference from Huawei HCIE-Datacom Documentation:
* Huawei MPLS LDP Configuration Guide - Liberal vs. Conservative Label Retention
* HCIE-Datacom Training Material - Label Advertisement in MPLS Networks