ic routing encapsulation) or the Layer 2 Tunnelling Protocol. The payload is carried in a tunnel that exists between two end points. Tunnelling provides communication between two subnetworks that have discontinuous IP addresses. It allows subscribers to connect to remote PDNs without publishing the user IP addresses on all intervening routers, etc. OSPF or RIP on the Gi interface are used for routing. The VRRP can be used to activate several physical routers and is seen as a single logical router, thereby eliminating a single point of failure for the router. It is transparent to the attached devices and hence no reconfiguration of the end devices are required. 4.13.5 Gp Interface Planning The interface of the mobile packet core (MPC) network to other PLMN networks is called the Gp interface. The Gp interface facilitates the roaming feature of the UE. This helps subscribers to use their home network services irrespective of the network coverage they are roaming. For this to happen, the two network operators either need to be connected directly or they should be connected via a centralised GPRS roaming exchange (GRX). Direct connectivity is possible through the leased lines or using tunnelling protocols, while centralised connectivity is done using a separate inter-PLMN backbone network with a root DNS. For roaming to take place, the contractual agreements between different network operators should be in place. If there are many operators, the complexity and the cost related to roaming will be high. However, the GRX concept can be used whereby global coverage through one connection is possible. This is better than the former option where all operators need to be connected to each other, as the number of links required to be maintained would be less. Planning engineers can start planning using the direct connectivity approach and then, as the network builds up, should move to a more centralised approach and end with a complete centralised solution. The IP address of packet core network elements must be unique betwee