Layer 3 VPN Attributes

Route distribution within a VPN is controlled through BGP extended community attributes. RFC 2547 defines the following three attributes used by VPNs:

• Target VPN—Identifies a set of sites within a VPN to which a provider edge (PE) router distributes routes. This attribute is also called the route target. The route target is used by the egress PE router to determine whether a received route is destined for a VPN that the router services.

  Figure 8 illustrates the function of the route target. PE Router PE1 adds the route target “VPN B” to routes received from the customer edge (CE) router at Site 1 in VPN B. When it receives the route, the egress router PE2 examines the route target, determines that the route is for a VPN that it services, and accepts the route. When the egress router PE3 receives the same route, it does not accept the route because it does not service any CE routers in VPN B.

• VPN of origin—Identifies a set of sites and the corresponding route as having come from one of the sites in that set.
• Site of origin—Uniquely identifies the set of routes that a PE router learned from a particular site. This attribute ensures that a route learned from a particular site through a particular PE-CE connection is not distributed back to the site through a different PE-CE connection. It is particularly useful if you are using BGP as the routing protocol between the PE and CE routers and if different sites in the VPN have been assigned the same autonomous system (AS) numbers.

Figure 8: VPN Attributes and Route Distribution

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