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Beginner

MPLS FEC and LIB

While reading MPLS basics, I have a query :-

FEC says providing the same behavior to same kind of packet that is allocating the same next hop, same egress interface.

AS book says - "In MPLS, the assignment of a particular packet to a particular FEC is done just once, as the packet enters the network. The FEC to which the packet is assigned is encoded as a short fixed length value known as a "label". "

That mean classifying the packets and assigning the labels as per the classification.

and for LIB book says - " The LIB functions in the control plane and is used by the label distribution protocol where IP destination prefixes in the routing table are mapped to next-hop labels that are received from downstream neighbors, as well as local labels generated by the label distribution protocol."

I am confused - FEC says classifying the packet and assigning the label and LIB says destination prefixes are mapped to labels.

If Destination prefixes need to mapped to labels then why do we need to classify the packets and why do we need FEC ?


I know that I am missing something because books area already reviewed by experts.

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Hall of Fame Cisco Employee

Hello Vishal,

If Destination prefixes need to mapped to labels

Technically, it is the other way round - labels are mapped to destination prefixes. First you have destinations, then you map labels to them on a per-hop basis.

why do we need to classify the packets

Because the packets need to flow through the network to a particular destination via a particular way. We need to make this determination of where the packets flow and which path they take as they enter the network. Therefore, as the packets come, we classify them into FECs, and once they are classified, we push labels on them and forward then through the Label Switched Paths we have chosen by this classifcation.

why do we need FEC

I have posted a similar answer here - perhaps it will help:

https://supportforums.cisco.com/message/3829452#3829452

Feel welcome to ask further!

Best regards,

Peter