TheTime-To-Live (TTL) fieldin MPLS is essential forpreventing infinite loopsandenhancing security.
MPLS TTL Field Functionality:
It prevents packets fromcirculating indefinitelyin the MPLS network.
It allowstraceroute (tracert) to identify MPLS hopswhen TTL propagation is enabled.
Explanation of Correct Answers:✅B. The processing of IP TTL copy hides the LSR in an MPLS domain, improving security.
IfTTL propagation is disabled, MPLS routers do not decrease TTL, hidinginternal LSRsfrom traceroute, enhancing security.
✅C. MPLS provides two TTL processing modes:
Default Mode:The MPLS TTLcopies the IP TTL valuewhen an IP packet enters the MPLS network.
Uniform Mode:The ingress LERsets the MPLS TTL to 255, so it does not decrement.
✅D. MPLS encapsulation in frame mode supports the TTL field, while cell mode does not.
Frame Mode (Ethernet, PPP, etc.) supports TTL.
Cell Mode (ATM/MPLS) does not support TTL.
Incorrect Answer Explanation:❌A. If TTL copy is disabled, users can use the tracert function to view MPLS hops.
Wrong!If TTL propagation is disabled, tracertcannotsee LSRs inside the MPLS core network.
Reference from Huawei HCIE-Datacom Documentation:
Huawei MPLS Technology Guide – TTL Field & Loop Prevention
HCIE-Datacom MPLS VPN Configuration Guide – Traceroute in MPLS Networks
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