2. Sites and CEs

2. Sites and CEs

From the perspective of a particular backbone network, a set of IP systems may be regarded as a "site" if those systems have mutual IP interconnectivity that doesn't require use of the backbone. In general, a site will consist of a set of systems that are in geographic proximity. However, this is not universally true. If two geographic locations are connected via a leased line, over which Open Shortest Path First (OSPF) protocol [OSPFv2] is running, and if that line is the preferred way of communicating between the two locations, then the two locations can be regarded as a single site, even if each location has its own CE router. (This notion of "site" is topological, rather than geographical. If the leased line goes down, or otherwise ceases to be the preferred route, but the two geographic locations can continue to communicate by using the VPN backbone, then one site has become two.)

A CE device is always regarded as being in a single site (though as we shall see in Section 3.2, a site may consist of multiple "virtual sites"). A site, however, may belong to multiple VPNs.

A PE router may attach to CE devices from any number of different sites, whether those CE devices are in the same or in different VPNs. A CE device may, for robustness, attach to multiple PE routers, of the same or of different service providers. If the CE device is a router, the PE router and the CE router will appear as router adjacencies to each other.

While we speak mostly of "sites" as being the basic unit of interconnection, nothing here prevents a finer degree of granularity in the control of interconnectivity. For example, certain systems at a site may be members of an intranet as well as members of one or more extranets, while other systems at the same site may be restricted to being members of the intranet only. However, this might require that the site have two attachment circuits to the backbone, one for the intranet and one for the extranet; it might further require that firewall functionality be applied on the extranet attachment circuit.