Vprn Service Overview - Alcatel-Lucent 7705 Service Manual

VPRN Service Overview

VPRN Service Overview
RFC 2547bis, an extension of RFC 2547, 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 that ensures that routes from
different VPNs remain distinct and separate, even if two VPNs have an overlapping address
space. Within a particular VPN, the PE routers distribute route information from and to the
CE routers. 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 an
individual 7705 SAR, a single label is assigned to (advertised for) all routes in a VPN. A
VRF lookup is used to determine the egress interface for a packet.
Before a customer data packet travels across the service provider's backbone network, it is
encapsulated with the MPLS label that corresponds, in the customer's VPN, to the route that
best matches the packet's destination address. That label (called the inner label) is the label
that was advertised from the destination 7705 SAR, as described in the previous paragraph.
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 its VPRN association. Thus, the backbone core routers do not need to know the
VPN routes.
Figure 41
"Red" and "Green") attached to PEs. The core routers are labeled "P".
Page 412
shows an example of a VPRN network diagram, showing two VPNs (labeled
7705 SAR OS Services Guide