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    MPLS Label Distribution Methodology

    The JunosE implementation of MPLS supports the following methods of label distribution:

    • Downstream-on-demand, ordered control with RSVP-TE
    • Downstream-unsolicited, independent control or ordered control with LDP; ordered control is the default. BGP accepts only downstream-unsolicited, ordered control

    Downstream-on-demand means that MPLS devices do not signal a FEC-to-label binding until requested to do so by an upstream device. Upstream is the direction toward a packet’s source; the ingress node in an MPLS domain is the farthest possible upstream node. Downstream is the direction toward a packet’s destination; the egress node in an MPLS domain is the farthest possible downstream node. The egress node is sometime referred to as the tunnel endpoint.

    Downstream-on-demand conserves labels in that they are not bound until they are needed and the LSR receives label mappings (also known as label bindings) from a neighbor that is the next hop to a destination; it is used when RSVP is the signaling protocol.

    Ordered control means that an LSR does not advertise a label for a FEC unless it is the egress LSR for the FEC or until it has received a label for the FEC from its downstream peer. In this manner the entire LSP is established before MPLS begins to map data onto the LSP, preventing inappropriate (early) data mapping from occurring on the first LSR in the path.

    An LSR is an egress LSR for a FEC when the FEC is its directly attached interface or when MPLS is not configured on the next-hop interface.

    In Figure 1, LSR A sends a label request to LSR C. Before LSR C responds, it sends its own request to LSR D. LSR D in turn makes a request for a label to LSR F. When LSR F returns an acceptable label to LSR D, that label is for use only between LSRs D and F. LSR D sends a label back to LSR C that this pair of LSRs will use. Finally, LSR C sends back to LSR A the label that they will use. This completes the establishment of the LSP.

    Downstream-unsolicited means that MPLS devices do not wait for a request from an upstream device before signaling FEC-to-label bindings. As soon as the LSR learns a route, it sends a binding for that route to all peer LSRs, both upstream and downstream. Downstream-unsolicited does not conserve labels, because an LSR receives label mappings from neighbors that might not be the next hop for the destination; it is used by BGP or LDP when adjacent peers are configured to use the platform label space.

    Figure 1: LSP Creation, Downstream-on-Demand, Ordered Control

    LSP Creation, Downstream-on-Demand, Ordered

    Independent control means that the LSR sending the label acts independently of its downstream peer. It does not wait for a label from the downstream LSR before it sends a label to its peers. When an LSR advertises a label to an upstream neighbor before it has received a label for the FEC from the next-hop neighbor, the LSP is terminated at the LSR. Traffic for the destination cannot be label-switched all the way to the egress LSR. If no inner label is present, then the traffic is routed instead of switched.

    In Figure 2, LSR D learns a route to some prefix. LSR D immediately maps a label for this destination and sends the label to its peers, LSR B, LSR C, LSR E, and LSR F. In the topology-driven network, the LSPs are created automatically with each peer LSR.

    Figure 2: LSP Creation, Downstream-Unsolicited, Independent Control

    LSP Creation, Downstream-Unsolicited,
Independent Control

    Published: 2014-08-18