Configuring LDP FEC Deaggregation

Beginning with JunosE Release 8.1.0, LDP routers running JunosE Software employ LDP FEC aggregation by default. FEC aggregation means that when an LDP egress router advertises multiple prefixes, all the prefixes are members of the same FEC. Only a single label is advertised for this FEC. LDP maintains this aggregation as the advertisement traverses the network, if possible.

Consider the topology shown in Figure 62.

Figure 62: FEC Aggregation and Equal-Cost Paths

FEC Aggregation and Equal-Cost Paths

In this example, LSR 2 uses FEC aggregation, but LSR 3 does not. Consequently, LSR 2 advertises the single label e, mapped to a FEC that includes both prefixes, 10.10.22.0/24 and 10.43.12.0/24.

In contrast, LSR 3 has two FECs, one for 10.10.22.0/24 and one for 10.43.12.0/24. A separate label is bound to each FEC. LSR 3 advertises label h for one FEC and label w for the other FEC.

LSR 2 and LSR 3 are downstream routers for LSR 1. LSR 1 does not aggregate. Instead, LSR 1 advertises label d for 10.10.22.0/24 and label x for 10.43.12.0/24.

To configure MPLS LDP FEC deaggregation to bind each prefix on the current virtual router to a separate label:

If you configure MPLS LDP FEC deaggregation to bind a separate label to each prefix on a virtual router, the default behavior is for the LDP egress router to advertise the implicit null label in the label mapping message that it sends to its upstream neighbor. If multiple labels are configured for the LDP egress router and FEC aggregation is configured, when you modify the label advertisement method to be FEC deaggregation, the egress router advertises the implicit null label and does not use separate labels with each prefix. With FEC deaggregation configured, the egress router's upstream neighbor performs a penultimate hop pop (PHP) and the implicit null label never appears in the encapsulation. If you configure MPLS LDP FEC deaggregation, the default advertised label is label 3 (implicit null label). In such a scenario, the penultimate-hop router removes the label and sends the packet to the egress router.

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