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JNCIE-SP: MPLS and Label Distribution

JNCIE-SP · Work through your exam, one topic at a time.

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MPLS and Label Distribution

What this domain tests

MPLS questions test label distribution with LDP, traffic engineering with RSVP-TE, forwarding-plane behavior, and label stacking. You must distinguish the control plane (LDP/RSVP) from the forwarding tables (inet.3 and mpls.0).

Core rules

  • LDP advertises a label for each FEC, typically loopbacks and connected prefixes. Enable it on core interfaces and the loopback: set protocols ldp interface ge-0/0/0.0 and set protocols ldp interface lo0.0. MPLS must also be enabled on the same interfaces.
  • LDP uses downstream unsolicited distribution and liberal label retention. It does not do traffic engineering.
  • RSVP-TE LSPs are configured under protocols mpls label-switched-path; bandwidth, priority setup hold, fast-reroute, auto-bandwidth, and optimize-timer tune them. CSPF uses the TED and prunes links that lack bandwidth or fail admin-group constraints.
  • The forwarding plane pushes a transport label at ingress, swaps at transit, and pops at the penultimate hop (PHP). The egress advertises implicit-null (label 3); explicit-null keeps a label for QoS.
  • inet.3 holds MPLS routes; mpls.0 is the label forwarding table. VPN and VC services stack an inner service label under the transport label.

Worked example

An LSP from PE1 to PE2 must reserve 100 Mbps and use a strict path through 10.0.0.1:

set protocols mpls label-switched-path TO-PE2 to 10.0.0.2
set protocols mpls label-switched-path TO-PE2 bandwidth 100m
set protocols mpls label-switched-path TO-PE2 primary VIA-R1
set protocols mpls path VIA-R1 10.0.0.1 strict

Reasoning: CSPF computes the path from the TED, excluding links with less than 100 Mbps unreserved bandwidth. The strict hop forces the first router. Verify with show mpls lsp extensive, which shows the actual path and reservation.

Traps

  • Enabling LDP but not MPLS on a core interface breaks label forwarding.
  • An RSVP LSP still needs a route to the egress; no-cspf uses the IGP path without bandwidth reservation.
  • PHP means the penultimate router pops the transport label; do not expect a label on the final hop unless explicit-null is configured.
  • LDP and RSVP can coexist, but only RSVP LSPs provide TE.