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A study about MPLS label

January 26, 2010

A study about MPLS label

Well Thanks for following the various articles in Networks-Baseline Blogs. Today we are come up with one of the most interesting article in the MPLS Service provider and that is called as MPLS Labels.


Basic MPLS network
Fig-1.1- Basic MPLS network

Before understanding the concept of Labels, everyone should know about the MPLS, that how it works and what are the capabilities of the MPLS in the service domain. Label is the part of the MPLS domain where information is transferred from one device to another device by the exchange of the labels.

MPLS Label Format

MPLS uses 32 bit of label field with the information like TTL, Experimental bit and bottom of stack. Let me show you the architecture of the MPLS label.
Basic MPLS network
Fig 1.1-MPLS Label Format
As per the above picture you can understand the size of the label field in the MPLS with all the information.
  • Label : 20 Bit field
  • EXP : 3 bit of Experimental field
  • Stack : 1 bit of bottom of Stack
  • TTL : 8 Bit of TTL Field
Now question is there, who is assigning labels in the MPLS domain ?
well, answer to the question is LDP protocol. LDP called as label distribution protocol used in the MPLS domain to assign the specified label for the specified routes in the MPLS domain. The prefix to label bindings are built and stored in the LIB (label information base), control plane, which is then used to create the LFIB (Label Forwarding Information Base) data or forwarding plane. The lookups are actually done in the LFIB, not the FIB.

So there are 3 terms you heard and that is LIB, FIB and LFIB, What is the difference between these 3 terms ?
So below is the answer to your question 

LIB is control plane - built by LDP, RSVP, etc based on entries in the RIB (routing information base - also control plane, built by OSPF, connected, static, etc). Maps all the entries in the RIB to an appropriate label
LFIB is forwarding plane. These are the entries that are used for forwarding lookup. Derived from the FIB (forwarding information base, which is derived from RIB).
FIB, along with the adjacency table is what comprises CEF.
⭐Note: Please don't include the labels that is learnt by RSVP, that is different concept and will be covered in MPLS traffic engineering.
MPLS Labels Mechanism
Fig 1.2 - MPLS Labels Mechanism

 The above diagram shows you the basic mechanism regarding the MPLS labels in the MPLS domain and how we can check with the commands as well. Please make yourself aware that all labels are locally significant and generated by router in the MPLS domain with the help of LDP protocol. Also make sure to enable the MPLS services on the router you need to enable the CEF command on the router first.