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JNCIS-SP: MPLS and Layer 3 VPNs: LDP, RSVP-TE, Label Stacking, and VRFs

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MPLS and Layer 3 VPNs: LDP, RSVP-TE, Label Stacking, and VRFs

Transport labels

LDP distributes labels bound to IGP routes and simply follows the IGP shortest path. Enable it on the loopback interface too - if the loopback is not an LDP interface, the LSP to a BGP next hop never forms and VPN routes stay unusable. RSVP-TE uses CSPF, explicit paths, bandwidth reservation and fast reroute, giving traffic engineering and sub-second protection that LDP cannot.

Label operations are push, swap and pop. The egress router advertises the implicit-null label (3) for routes it terminates, so the penultimate hop pops the transport label (PHP) and the egress receives a bare IP packet or a single VPN label.

Label stacking and VPN forwarding

A packet from CE1 to CE2 carries two labels:

  • Outer/transport label - pushed by the ingress PE, derived from LDP or RSVP toward the egress PE loopback.
  • Inner/VPN label - assigned by the egress PE to identify the VRF or the specific prefix.

The egress PE pops the transport label and uses the VPN label to choose the VRF table.

VRF configuration

A Junos VRF is routing-instance <name> instance-type vrf with interfaces, a route distinguisher and route targets. Key distinctions:

  • Route distinguisher (RD) only makes prefixes unique in MP-BGP; it plays no role in import decisions.
  • Route target (RT) is an extended community that decides which VPN routes are imported into which VRF (vrf-target or separate vrf-import/vrf-export).

Trap: mismatched route targets mean routes are advertised but never installed - the VRF stays empty while BGP shows the prefix as valid. vrf-table-label allocates a single per-VRF label rather than one per prefix.

MP-BGP uses family inet-vpn unicast; the NLRI is RD + prefix + label, with the PE loopback as next hop.