nd internal PE interfaces) uses an IGP and implements LDP as described earlier, then all the necessary LSPs between the PE routers will be provisioned automatically. However, we will see that this is not the only labeling of traffic needed in the network. The key protocol that enables the MPLS VPN is the Border Gateway Protocol (BGP). BGP is implemented on the PE LSRs with the external interface (EBGP) toward a CE; the other PEs are Internal BGP (IBGP) interfaces. For this application BGP version 4 is used but with multiprotocol extensions as specified in RFC 2283. The multiprotocol extensions speci- fied for BGP have nothing to do with the term "multiprotocol" as used in MPLS. Instead, the multiprotocol extensions to BGP allow multiple address spaces to be used, and each address space can be given an identifier. This 64-bit identifier is often written ASN:x, where ASN is the Autonomous System Number of the provider (a globally assigned number by IANA) and x is a number assigned by the provider to uniquely identify each customer. This identifier is termed a Routing Designator (RD), and one is configured for each customer, requiring that a corresponding PSFT be enabled for each customer connected to a PE. As with standard BGP (RFC 17171), each PE has to be configured to peer with every other PE as an IBGP interface. Configuration of which external sites should be "con- nected" together is implemented by BGP route targets specified in RFC 4364; each route target is a BGP extended community. The route targets are configured for each RD on each PE. BGP will only import or export routes that share a common routing target. As an example of a configuration, consider how PEA would be set up to allow connectivity between site A and site B, which are Customer-1 sites. First, PEA knows to speak to PEB through manual BGP configuration. Next, both PEA and PEB need to be assigned a common RD for the address space of customer 1; this could be called 1:1. To make PEA and PEB share the routes for RD 1:1, they are each configured 