Wednesday, April 11, 2012

MP-BGP & Routing Distinguishers


  • Routes learned from the CE router are advertised to other PE routers uses the IBGP from all the routes, from all the different VRFs.
  • If use normal BGP is used, may overlapping of prefixes will be occurred.
  • MPLS deals this problem by
    • Add another number in front of the original BGP NLRI.
    • Each different number can represent a different customer.
    • To do this MPLS uses the MultiProtocol BGP.
  • MP BGP  allows re-define the NLRI filed in BGP updates.
  • This re-defination allows for an additional variable-length umber, called Address family
    • This address family added at, in front of the prefix. 
  • MPLS RFC 4363, "BGP/MPLS IP Virtual Private Networks(VPNs)," defines a specific new address family to support IPv4 MPLS VPNs--named as an MP-BGP address family called  Route Distinguishers (RDs)
  • RDs allow BGp to advertise & distinguish between duplicate IPv4 prefixes.
  • The concept is simple:
    • Advertise each NLRI as the traditional IPv4 prefix, but add another number (the RD)
      • RD uniquely identifies the route.
  • In the new NLRI format, called VPN-V4, has 2 parts:
      • 64-bit RD
      • 32-bit IPv4 prefix
    • example: 1:111:10.2.2.0/24
  • Every VRF must be configured with an RD.

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