Virtual Routing And Forwarding Tables; Vpn Routing Information: Distribution; Bgp Distribution Of Vpn Routing Information - Cisco CRS Configuration Manual

How MPLS L3VPN Works
• Exchanges routing updates with the CE router
• Translates the CE routing information into VPN version 4 (VPNv4) and VPN version 6 (VPNv6) routes.
• Exchanges VPNv4 and VPNv6 routes with other PE routers through the Multiprotocol Border Gateway
Protocol (MP-BGP)

Virtual Routing and Forwarding Tables

Each VPN is associated with one or more VPN routing and forwarding (VRF) instances. A VRF defines the
VPN membership of a customer site attached to a PE router. A VRF consists of the following components:
• An IP version 4 (IPv4) unicast routing table
• A derived FIB table
• A set of interfaces that use the forwarding table
• A set of rules and routing protocol parameters that control the information that is included in the routing
table
These components are collectively called a VRF instance.
A one-to-one relationship does not necessarily exist between customer sites and VPNs. A site can be a member
of multiple VPNs. However, a site can associate with only one VRF. A VRF contains all the routes available
to the site from the VPNs of which it is a member.
Packet forwarding information is stored in the IP routing table and the FIB table for each VRF. A separate
set of routing and FIB tables is maintained for each VRF. These tables prevent information from being
forwarded outside a VPN and also prevent packets that are outside a VPN from being forwarded to a router
within the VPN.

VPN Routing Information: Distribution

The distribution of VPN routing information is controlled through the use of VPN route target communities,
implemented by BGP extended communities. VPN routing information is distributed as follows:
• When a VPN route that is learned from a CE router is injected into a BGP, a list of VPN route target
extended community attributes is associated with it. Typically, the list of route target community extended
values is set from an export list of route targets associated with the VRF from which the route was
learned.
• An import list of route target extended communities is associated with each VRF. The import list defines
route target extended community attributes that a route must have for the route to be imported into the
VRF. For example, if the import list for a particular VRF includes route target extended communities
A, B, and C, then any VPN route that carries any of those route target extended communities—A, B, or
C—is imported into the VRF.

BGP Distribution of VPN Routing Information

A PE router can learn an IP prefix from the following sources:
• A CE router by static configuration
Cisco IOS XR Virtual Private Network Configuration Guide for the Cisco CRS Router, Release 6.1.x
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Implementing MPLS Layer 3 VPNs