MPLS Label Parameters

MPLS Label paramters;


  • Per-interface label space
    • Labels are assigned unique to specific input interface
    • Same labels can be used on another input interface with different meaning.
    • Label forwarding based on incoming interface and label. Hence, label spoofing can be avoided.
    • Used in cell-mode MPLS (LC-ATM)
    • Per-platform Label space
      • Labels are unique for whole router.
      • Label forwarding based solely on labels alone.
      • Used in Frame-mode MPLS
      • Reduces number of LDP session to one, in case of multiple links between 2 LSRs


    • Unsolicited downstream
      • Labels are advertised to all LDP neighbors.
      • Usually used in Frame-mode MPLS
    • Downstream-on-demand
      • Upstream LSR, request a label from downstream LSR and only then, downstream LSR will assign a label and distribute to upstream LSR.
      • Usually used in Cell-mode MPLS.
    • Both methods can co-exist in MPLS network as label distribution is hop-by-hop process. For Eg; in MPLS TE, head-end LSR sends RSVP PATH message to tail-end and only then, tail-end router assigns a label for TE (usually label-3 due to PHP).


    • Independent control;
      • LSRs will assign a local label for a prefix as soon as the prefix is available in routing table via IGP.
      • Used in Frame-mode MPLS
    • Ordered Control;
      • LSRs will assign a local label only when it has a next-hop label
      • Mandatory in Cell-mode MPLS


    • Liberal Retention Mode;
      • LSRs stores all the labels received from LDP neighbors.
      • Memory required for storing all labels. Not a big deal in present scenario.
      • Convergence takes place immediately.
      • Used in Frame-mode MPLS
    • Conservative retention Mode;
      • LSRs stores only the next-hop label.
      • Downstream-on-demand distribution needs to take place during convergence.
      • Used in Cell-mode MPLS.

    In Brief, Routers with frame interface uses per-platform, unsolicited downstream, independent control and liberal retention mode.

    ATM switches, uses per-interface, downstream-on-demand, ordered control and conservation retention mode.

    Routers with ATM interfaces (Edge ATM LSRs)  uses per-interface, downstream-on-demand, independent control, liberal or conservative retention mode.

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