A label edge router (LER, also known as edge LSR) is a router that operates at the edge of an MPLS network and acts as the entry and exit points for the network. LERs push an MPLS label onto an incoming packet and pop it off an outgoing packet. MPLS labels are added between the Layer 2 and the Layer 3 header in the packets (-> Therefore MPLS labels are added before Layer 3 header). There are no limit on the number of labels in a stack. A label is a short, four-byte, fixed-length, locally-significant identifier which is used in order to identify a Forwarding Equivalence Class (FEC). The label which is put on a particular packet represents the FEC to which that packet is assigned. LDP uses TCP as a reliable transport for sessions. Each TCP connection has only one LDP session. Reference: https://www.cisco.com/c/en/us/support/docs/multiprotocol-label-switching- mpls/mpls/4649-mpls-faq-4649.html