• How it works?
    • Separation of customer routing information.
      • VRF
      • Different customers have different routing tables.
      • IGP/BGP run inside the VRF between the customer and SP.
    • Exchange of customer’s routing info inside SP.
      • MP-BGP through the SP network.
      • Traffic is label switched towards BGP next-hops.
  • VRF Lite vs. MPLS VPNs
    • In VRF Lite all devices in transit path must carry all routes in all VRF tables.
    • In MPLS VPNs only PE routers need customer routes
    • Accomplished via the following:
      • VPNv4 BGP
        • Route Distinguisher + Prefix makes VPN routes globally unique.
      • MPLS VPN Tag/Label
        • P routers only need to know how to reach BGP next-hop.
        • BGP free core logic.
  • High Level
    • Establish Label Switched Path (LSP) between PEs.
      • IGP and LDP
    • Exchange routes with customer.
      • PE-CE IGP or BGP
    • Exchange customer routes between PEs.
      • iBGP and MPLS VPN labels
    • Label Switch from PE to PE.
      • Data follows the IGP and LDP transport label.
  • Multi-protocol BGP
    • How do PE routers exchange VRF info?
      • RFC 4364 MPLS IP VPNs
    • MP-BGP Defines AFI 1 and SAFI 128 as VPN-IPv4 or VPNv4
      • 8 byte Route Distinguisher (RD)
        • Unique per VPN or per VPN site.
        • ASN:nn or IP-address:nn
      • 4 byte IPv4 address
        • Unique per VPN
      • Implies globally unique routes.
    • VPNv4 includes MPLS VPN label
  • NLRI Format
    • VPNv4 NLRI main attributes include…
      • 8 byte RD
        • Unique per VPN or per VPN site.
        • ASN:nn or IP-address:nn
      • IPv4 prefix and length
        • Unique per VPN because of RD
      • Next hop
      • MPLS VPN label
    • Regular BGP attributes stay the same.
  • VPNv4 Routes
    • Route Distinguisher used solely to make route unique.
      • Allows for overlapping IPv4 addresses between customers.
    • New BGP extended community ‘route-target’ used to control what enters/exits VRF table.
      • export route-target
        • What routes will go from VRF into BGP
      • import route-target
        • What routes will go from BGP into VRF
    • Allows granular control over what sites have what routes.
  • Route Distinguisher vs. Route Target
    • Route Distinguisher
      • Makes route unique
      • Only one RD per VPNv4 route.
    • Route Target
      • Controls the route’s VPN memberships
      • Can be multiple RTs per VPNv4 route.
  • Route Target
    • 8 byte field
      • RFC 4360
    • Format similar to route distinguisher
      • ASN:nn or IP-address:nn
    • VPNv4 speakers only accept VPNv4 routes with a route-target matching a local VRF
      • Some exceptions, eg. route-reflectors.
    • VPNv4 routes can have more than one RT
      • Allows complex VPN topologies.
      • Full mesh
      • Hub and spoke
  • Transport label vs. VPN label
    • L3VPN needs at least 2 labels to deliver traffic.
      • can be more with applications like MPLS TE, FRR, etc.
    • Transport label
      • Tells SP core routers which PE traffic is destined for.
        • Who is exit point.
      • Typically derived from LDP
        • Sometimes called IGP label.
    • VPN Label
      • Tells PE router which CE traffic is destined for.
      • Derived from VPNv4 advertisements of PEs.
    • In general, VPN label used for final destination/VRF connectivity and Transport label used for label switching through SP core.

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