MPLS

Enabling MPLS

MPLS label processing is per-interface. Enable it on each interface that should accept labeled packets:

interface {
    ethernet eth0 {
        unit 0 {
            mpls {
                enable true
            }
        }
    }
}

This sets net.mpls.conf.eth0.input=1. The global label-table size is the net.mpls.platform_labels sysctl. It defaults to 0, which disables MPLS entirely, so ze writes the full 20-bit space (1048575) once, immediately before it programs its first label. An operator value set in the sysctl {} block is never overwritten: only a table reading exactly 0 is repaired.

The Linux kernel must supply MPLS forwarding, either through the mpls_router and mpls_iptunnel modules or built in with CONFIG_MPLS_ROUTING and CONFIG_MPLS_IPTUNNEL. ze's own appliance kernel builds both in, so it loads no module. MPLS is an enrolled kernel capability: ze probes for the AF_MPLS table rather than for the module list, because a built-in kernel lists no module.

When the configuration asks the kernel FIB to forward MPLS and /proc/sys/net/mpls/platform_labels does not exist, ze doctor reports doctor-mpls-unavailable at error severity, ze refuses to start, and ze config validate fails. There is no operator override. A daemon that advertises label forwarding it cannot perform blackholes the traffic it attracted.

A probe ze could not read is a different answer. It reports doctor-mpls-unknown at warning severity and ze starts, because refusing on a question ze could not ask would stop a router whose kernel is fine.

A VPP or P4 FIB backend is never judged on the kernel's AF_MPLS table. The fib { kernel { } } block is what activates the plugin that programs kernel labels, and it is that plugin that carries the requirement.

Inspecting the forwarding table

ze show mpls forwarding              # all installed MPLS entries
ze show mpls forwarding limit 500    # cap the response size

Each row reports the incoming label (in-label), the operation (swap or pop), any outgoing out-labels, the next-hop, and the egress device. The data is read directly from the kernel AF_MPLS routing table.

Metrics

• ze_fibkernel_mpls_routes_installed -- current MPLS labeled-route count.
• ze_fibkernel_mpls_installs_total -- MPLS routes successfully programmed.

Label distribution

Two label-distribution protocols are available (both experimental):

• LDP (RFC 5036) for IGP-shortest-path label distribution.
• RSVP-TE (RFC 3209) for traffic-engineered explicit-path LSPs with bandwidth reservation -- see RSVP-TE.

LDP configuration

ldp {
    lsr-id 10.0.0.1
    transport-address 10.0.0.1
    interfaces eth0
    interfaces eth1
}

• lsr-id -- this router's LSR identifier (IPv4 address format).
• transport-address -- address advertised for the LDP TCP session.
• interfaces -- a list entry per interface on which LDP Hello discovery runs (repeat the leaf for each interface).
• hello-interval / hello-hold-time / keepalive-time -- optional soft-state timers (defaults 5s / 15s / 60s).

keepalive-time is the KeepAlive Time ze proposes in the Initialization message of each new session (RFC 5036 section 3.5.3). The two LSRs keep the lower of the two proposals. Ze then sends a KeepAlive every third of the negotiated value, and it closes the session when no PDU arrives inside three times that value. The proposal is exchanged one time, when the session starts, so a change to this leaf applies to the sessions that open after it and leaves an established session on the value it negotiated.

Inspect LDP state with show ldp neighbor (session state, transport address) and show ldp binding (FEC-to-label bindings). A label binding learned from a neighbor programs an ingress push entry in the kernel MPLS FIB toward that neighbor.