Vprn Service Overview - Alcatel-Lucent 7750 SR OS Service Manual

VPRN Service Overview

VPRN Service Overview
RFC2547bis is an extension to the original RFC 2547, which details a method of distributing
routing information and forwarding data to provide a Layer 3 Virtual Private Network (VPN)
service to end customers.
Each Virtual Private Routed Network (VPRN) consists of a set of customer sites connected to one
or more PE routers. Each associated PE router maintains a separate IP forwarding table for each
VPRN. Additionally, the PE routers exchange the routing information configured or learned from
all customer sites via MP-BGP peering. Each route exchanged via the MP-BGP protocol includes
a Route Distinguisher (RD), which identifies the VPRN association.
The service provider uses BGP to exchange the routes of a particular VPN among the PE routers
that are attached to that VPN. This is done in a way which ensures that routes from different VPNs
remain distinct and separate, even if two VPNs have an overlapping address space. The PE routers
distribute routes from other CE routers in that VPN to the CE routers in a particular VPN. Since
the CE routers do not peer with each other there is no overlay visible to the VPN's routing
algorithm.
When BGP distributes a VPN route, it also distributes an MPLS label for that route. On a SR-
Series, a single label is assigned to all routes in a VPN.
Before a customer data packet travels across the service provider's backbone, it is encapsulated
with the MPLS label that corresponds, in the customer's VPN, to the route which best matches the
packet's destination address. The MPLS packet is further encapsulated with either another MPLS
label or GRE tunnel header, so that it gets tunneled across the backbone to the proper PE router.
Each route exchanged by the MP-BGP protocol includes a route distinguisher (RD), which
identifies the VPRN association. Thus the backbone core routers do not need to know the VPN
routes.
Figure 52
Page 856
displays a VPRN network diagram.
7750 SR OS Services Guide