The SEEREN physical and logical network topologies are depicted in Figure 12-13 and Figure 12-14, respectively. The MPLS-enabled core network infrastructure, which is provided by a consortium of operators in SE Europe, has Points of Presence (PoPs) in all the SE European capital cities. Interconnection between the SEEREN NRENs is achieved with the Carrier supporting Carrier (CsC) technique (see Figure 12-15), which enables an MPLS VPN-based service provider, called the carrier provider, to allow other IP (or MPLS) service providers, called customers carriers (SEEREN NRENs1), to use a segment of its backbone network.

The CsC technique offers to the SEEREN NRENs multiple technical and economical advantages. SEEREN NRENs do not need to deploy, operate and maintain an international backbone infrastructure but only need to exchange minimal routing information with its customers, e.g. only 15 routes are needed for the entire SEEREN network, even in cases where several instances of the entire Full Internet Routing Table is exchanged over the Provider’s network. This allows SEEREN NRENs to define their routing policy (eBGP sessions) transparently over the carrier provider’s network and the carrier provider to minimize the routing information stored in the VRF tables.