BGP/MPLS VPN Components Overview

If you have specified the VPN-IPv4 address family, you can configure virtual private networks across an IP backbone. BGP carries routing information for the network and MPLS labels, whereas MPLS transports the data traffic. Figure 69 shows a typical scenario.

The service provider backbone comprises two types of routers:

PE routers are situated at the edge of the service provider core and connect directly to customer sites. These routers must run BGP-4, including the BGP/MPLS VPN extensions. They must also be able to originate and terminate MPLS LSPs. (See Configuring MPLS, for more information.)

P routers connect directly to PE routers or other P routers and do not connect directly to customer sites. These routers must be able to switch MPLS LSPs—that is, they function as MPLS label-switching routers (LSRs) and might function as label edge routers (LERs). Running BGP-4 on the P routers is not necessary to be able to exchange routing information for VPNs. You might run BGP-4 on the core routers for other reasons, such as exchanging routing information for the public Internet or implementing route reflectors. The P routes do not need to contain any information about customer sites.

PE routers communicate with customer sites through a direct connection to a customer edge (CE) device that sits at the edge of the customer site. The CE device can be a single host, a switch, or, most typically, a router. When the CE device is a router, it is a routing peer of all directly connected PE routers, but it is not a routing peer of CE routers at any other site. The link between the CE router and the PE router can employ any type of encapsulation. Using MPLS is not necessary. In Figure 69, each PE router connects to multiple CE routers and at least one P router. Although only one customer site is shown, each CE router lies within a customer site.

Figure 69: BGP/MPLS VPN Scenario

BGP/MPLS VPN Scenario

A customer site is a network that can communicate with other networks in the same VPN. A customer site can belong to more than one VPN. Two sites can exchange IP packets with each other only if they have at least one VPN in common.

Each customer site that is connected to a particular PE router is also associated with a VPN routing and forwarding instance (VRF). As shown in Figure 70, each VRF has its own forwarding table distinct from that of other VRFs and from the virtual router’s global forwarding table.

Figure 70: BGP/MPLS VPN Components

BGP/MPLS VPN Components

A given VRF’s forwarding table includes only routes to sites that have at least one VPN in common with the site that is associated with the VRF. For example, in Figure 70, the forwarding table in VRF B stores routes only to sites that are members of at least one of the VPNs to which Customer Site 3 belongs.

VRFs exist within the context of a virtual router (VR). A given virtual router can have zero or more VRFs, in addition to its global routing table (which is not associated with any VPN, CE router, or customer site). A router can support up to 1000 forwarding tables; that is, up to a combined total of 1000 VRs and VRFs.

You assign one or more interfaces or subinterfaces to a given VRF. If multiple customer sites are members of the same set of VPNs, they can share a VRF—that is, you do not need to create a specific VRF for each customer site. In Figure 70, Customer Sites 1 and 2 share VRF A; both sites belong to the same set of VPNs. The router looks up a packet’s destination in the VRF associated with the interface on which the packet is received. The VRFs are populated by BGP while it learns routes from the VPN. If a customer site is a member of multiple VPNs, the routes learned from all those VPNs populate the VRF associated with the site.

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