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Q41Layer 3 Technologies

Match the LDP features on the left to the descriptions on the right. <DragDrop items={["implicit null label","explicit null label","inbound label binding filtering","entropy label"]} slots={[{"answer":"entropy label","id":"slot1","label":"provides ways of improving load balancing by eliminating the need for DPI at transit LSRs"},{"answer":"implicit null label","id":"slot2","label":"LSR receives an MPLS header with the label set to 3"},{"answer":"explicit null label","id":"slot3","label":"packet is encapsulated in MPLS with the option of copying the IP precedence to EXP bits"},{"answer":"inbound label binding filtering","id":"slot4","label":"controls the amount of memory used to store LDP label bindings advertised by other devices"}]} explanation={"An entropy label supplies extra hashing information so transit LSRs can load-balance MPLS traffic without deep packet inspection. MPLS label 3 is the IPv4 implicit-null label. An explicit-null label retains the MPLS header and supports carrying the IP precedence in the EXP/TC field. Inbound label binding filtering restricts retained LDP bindings, reducing the memory consumed by bindings advertised by peers.\n\n**Learn more:** [MPLS LDP Entropy Label Support](https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mp_ldp/configuration/xe-16/mp-ldp-xe-16-book/mp-ldp-entropy.html) · [MPLS LDP Inbound Label Binding Filtering](https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mp_ldp/configuration/15-s/mp-ldp-15-s-book/mp-ldp-inbound-filtr.html)"} reuse={false} />

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Discussion · 5
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That's correct. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mp_ldp/configuration/xe-16-6/mp-ldp-xe-16-6-book/mp-ldp-inbound-filtr.html https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mp_ldp/configuration/xe-16/mp-ldp-xe-16-book/mp-ldp-entropy.html https://www.ipspace.net/kb/tag/MPLS/Implicit_Explicit_NULL.html
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Provides ways to improve load balancing by removing the need for DPI at transit LSRs LSRs receives an MPLS header with the label set to 3 Packets are encapsulated in MPLS with the option of copying the IP priority into the EXP bit Controls how much memory is used to store LDP label bindings advertised by other devices
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In my opinion, the explanation about implicit and explicit is NOT correct or is really badly written. Explicit null>> MPLS label is popped by next-to-last LSR and "normal" IP packet gets forwarded to last LSR (egress LSR) Implicit null label: MPLS label is not taken off by next-to-last LSR, this retains MPLS EXP (QOS bits) so they can still be used by egress LSR
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The value "Implicit NULL Label == 3" will only be communicated by the egress LSR through LDP. When encapsulation takes place, the next-to-last LSR knows it can pop the entire MPLS header as it has previously received the "implicit null value of 3 by means of LDP". That does mean that "LSR receives an MPLS header with the label set to 3" is actually incorrect.
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My first entry was wrong, see below for the correct text Implicit null>> MPLS label is popped by next-to-last LSR and "normal" IP packet gets forwarded to last LSR (egress LSR) Explicit null label: MPLS label is not taken off 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