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MPLS VPNs in real World

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I am new member on this forum. Can anybody guide me that in real world when ISP provides MPLS VPN to any of its customer for connecting its two different sites then how does it allocate the VPN number to this client's VPN for distinguishing it from that of other customers. Thanks.

Edited by ashar
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The service providers allocates RD (route distinguishers) and RT (route targets) on each customer.

They will configure a specific vrf for each customer.


The RT's and RD's are carried in MP-BGP on PE routers participating on any locations.

if it successful then, a PE from point A can learn the routes from a PE at point B.


The importing/exporting of routes from each PE is very important to ensure the customer routes are learned anywhere. The ISP is also cautious to configure the RT's correctly so that one customer routing will not leak to another customer routes.

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Refer to WAN Deployment guide from here


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RD (Route Distinguisher)

  • A VPN's routes are propagate d across a MPLS VPN network by MP-BGP. MP-BGP requires that the transported routes be unique .
  • An RD is a 64 -bit (8 -byte ) value prepended to a client's non -unique 32 -bit IPv4 address to produce a unique 9 6 -bit VPNv4 address.
  • An RD uniquely identifies a route , it does NOT identify a VPN.
  • An RD is locally significant to a route r b u t has global relevance


RT (Route-Target)

  • Is a 64 -bi t (8 -byte ) extended BGP community that is attached to a VPNv4 BGP route to indicate its VPN membership .
  • A certainn umber o f RTs can be attached to a single route , up to the BGP Update packet size of 4096 .
  • Export RTs
    • Are attached to a route when i t is converted in to a VPNv4 route .
    • Generally used to identify the VPN membership of routes .

    [*]Import RTs

    • Are used to select VPN v4 routes for insertion in to matching VRF tables .
    • On the receiving PE router, a route is imported in to a VRF only if at least one RT attached to the route matches at least one import RT configured in that VRF (route -map conditions must be met, i f configured).

  • An import or export map allows route control on a per-route basis.
  • RTs allow for more complex VPN designs like Hub -and-Spoke , Central Services, Extranet, Management VPNs, e tc.

RT/RD can be used i n one o f the foll owing formats :

  • ASN:nn - Autonomous System Number; where 'nn ' can be any number.
  • IP-ADD :nn - 4 -Octet Dotted Decimal format; where 'nn ' can be any number.


Edited by tussar01
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