300-410 Question 102

Community Discussion

Written by other learners. Comments can disagree with the marked answer — check the official documentation when in doubt.

• 1

  My first entry was incorrect, see below for the correct text Implicit null>> MPLS label is popped by next-to-last LSR and "normal" IP packet get's forwarded to last LSR (egress LSR) Explicit null label: MPLS label is not taken of by next-to-last LSR, this retains MPLS EXP (QOS bits) so they can still be used by egress LSR More info: https://www.networkworld.com/article/912436/cisco-subnet-understanding-mpls-explicit-and-implicit-null-labels.html https://community.cisco.com/t5/mpls/implicit-null-and-explicit-null/td-p/1496792

• 1

  In my opinion is the explanation about implicit and explicit NOT correct or really badly written. Explicit null>> MPLS label is popped by next-to-last LSR and "normal" IP packet get's forwarded to last LSR (egress LSR) Implicit null label: MPLS label is not taken of by next-to-last LSR, this retains MPLS EXP (QOS bits) so they can still be used by egress LSR

• 1

  The value "Implicit NULL Label == 3" will only be communicated by egress LSR by LDP. When encapsulation takes place on the next-to-last LSR knows it can pop entire MPLS header as it previously has received the "implicit null value of 3 by means of LDP". Above does mean that "LSR receives an MPLS header with the label set to 3" is actually incorrect.