MPLS VPN—VRF Selection

The VPN Routing and Forwarding (VRF) Selection feature allows a specified interface on a provider edge (PE) router to route packets to different Virtual Private Networks (VPNs) based on the source IP address of the packet. This feature is an improvement over using a policy-based router to route packets to different VPNs.

Feature Overview

The VRF Selection feature allows packets arriving on an interface to be switched into the appropriate VRF table based upon the source IP address of the packets. Once the packets have been "selected" into the correct VRF routing table, they are processed normally based upon the destination address and forwarded through the rest of the Multiprotocol Label Switching (MPLS) VPN.

In most cases, the VRF Selection feature is a "one way" feature; it works on packets coming from the end users to the PE router.

VRF Selection Process

The VRF Selection feature uses the following process to route packets from the customer networks to the PE router and into the provider network.

A two-table lookup mechanism is used at the ingress interface of the PE router to determine the routing and forwarding of packets coming from the customer networks, which use IP protocols, to the MPLS VPN networks, which use MPLS protocols.

• The first table, the VRF Selection table, is used to compare the source IP address of the packet with a list of IP addresses in the table. Each IP address in the table is associated with an MPLS VPN. If a match is found between the source IP address of the packet and an IP address in the VRF Selection table, the packet is routed to the second table (the VRF table) or the routing table for the appropriate VPN.

  If no match is found in the table for the source IP address of the packet, the packet will either be routed via the global routing table used by the PE router (this is the default behavior), or will be dropped. See the "Configuring a VRF to Eliminate Unnecessary Packet Forwarding" section for more information.

• The second table, the VRF table (also known as the VPN routing table), contains the virtual routing and forwarding information for the specified VPN and is used to forward the selected VPN traffic to the correct MPLS label switched path (LSP) based upon the destination IP address of the packet.

The VRF Selection process removes the association between the VPN and the interface and allows more than one MPLS VPN to be associated with the interface.

source: http://cisco.com