loyed in a way where the IP address configured to the PDN Connection is topologically anchored to a router other than the PGW. In this deployment use case the PGW effectively becomes a bridge/relay and all the IP level functionality is 'delegated' to this external router and the net- work behind it. So-called 'corporate APNs' or interworking with PDN are typically built like this. For the GPRS and a GGSN with Gn/Gp-based interfaces, 3GPP specifications also defined interworking with PDN using PPP [25]. It is not supported as such by EPC, since the required PDP Type PPP was deprecated in 3GPP Release-8. In this deployment option the GGSN takes the role of Layer 2 Tunneling Protocol (L2TP) Access Concentrator (LAC) and the external router has the role of a L2TP Network Server (LNS). The PPP packets to/from the UE are either relayed by the GGSN to/from the LNS or converted to some other protocol depending on the tunneling protocol between the GGSN and the external router. Generally speaking, there is no uniform 3GPP standard on how to accomplish the transport of the IP traffic between the UE and the external router in the case of corporate APNs, and how to make the PGW behave as a bridge. On the other hand, there is a description for PPP- and L2TP-based external router interworking scenario for the PDP Type PPP, which has been deprecated since 3GPP Release-8. From practical deployments and vendor implementations points of view, L2TP can be and is used with PDN Type IP. The PGW has the role of LAC and also performs the required protocol conversion from GTP to PPP, since there is no end-to-end PPP between the UE and the LNS. IPv6 in 3GPP Networks Addressing scope External IPIP / (Internet) RELAY RELAY RELAY IPsec / GTP-U GTP-U GTP-U Tunnel GTP-U Tunnel L2TP / UDP/IP UDP/IP Trans- Trans- UDP/IP UDP/IP LTE-Uu S5/S8 External first-hop eNodeB router (e.g. LNS) Figure 4.8 EPC and an external 'APN router' deployment. IP traffic between the UE and the external router has to be tunneled between the PGW and the external router