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    Controlling LDP Label Distribution

    By default, LDP advertises label mappings for all IGP prefixes to all LDP peers. In this case, mappings are not advertised for interface addresses. You can alternatively specify that LDP labels be distributed for a particular interface itself, in addition to the subnet that the interface is on. This behavior enables LSPs to be set up to the LSR configured with the interface address.

    When the LSR learns an IGP route and tries to decide whether to advertise a label for the destination to a particular LDP neighbor, it attempts to match the destination against a route access list specified by the mpls ldp advertise-labels command, in the order in which the commands were issued. The first match determines the action taken, and no further matching is attempted for that destination. If the destination matches, labels are advertised to peers subject to any specified neighbor address list. If either access list is not matched, the labels are not advertised.

    To create a filter that determines whether and where incoming (locally assigned) labels are distributed:

    • Issue the mpls ldp advertise-labels command one or more times:
      host1(config)#mpls ldp advertise-labels for net25 to euro3

    When you do not specify a toAccessList, the action is taken for all peers.

    Consider the following example configuration.

    host1(config)#mpls ldp advertise-labels for net25 to euro3 host1(config)#mpls ldp advertise-labels for boston1

    In this example, suppose the LSR receives a label for destination 10.10.11.12. If net25 specifies 10.10.11.12, then the access list action—permit or deny—is taken with the destination. If the action is permit, the peer that the label is advertised to is subject to the access list euro3. If net25 does not include 10.10.11.12, the LSR attempts to match it against boston1. If 10.10.11.12 is present in that access list, the specified action is taken for all peers. If boston1 does not include the destination, the label is not advertised to any peer.

    Published: 2014-08-18