242Nested VPN is flexible and easy to implement and can reduce the cost because a customer only needsto pay for one MPLS VPN to have multiple internal VPNs connected. Nested VPN provides diversifiedVPN networking methods for a customer, and allows for multi-level hierarchical access control over theinternal VPNs.HoVPNIn MPLS L3VPN solutions, PEs are the key devices, which provide the following functions:• User access. This means that the PEs must have a large amount of interfaces.• VPN route managing and advertising, and user packet processing, requiring that a PE must have alarge-capacity memory and high forwarding capability.Most of the current network schemes use the typical hierarchical architecture. For example, the MANarchitecture contains typically three layers, namely, the core layer, distribution layer, and access layer.From the core layer to the access layer, the performance requirements on the devices decrease while thenetwork expands.MPLS L3VPN, on the contrary, is a plane model where performance requirements are the same for all PEs.If a certain PE has limited performance or scalability, the performance or scalability of the whole networkis influenced.Due to the difference, you are faced with the scalability problem when deploying PEs at any of the threelayers. Therefore, the plane model is not applicable to the large-scale VPN deployment.To solve the scalability problem of the plane model, MPLS L3VPN must transition to the hierarchicalmodel.In MPLS L3VPN, hierarchy of VPN (HoVPN) was proposed to meet that requirement. With HoVPN, the PEfunctions can be distributed among multiple PEs, which take different roles for the same functions andform a hierarchical architecture.As in the typical hierarchical network model, HoVPN has different requirements on the devices atdifferent layers of the hierarchy.Implementation of HoVPNFigure 71 Basic architecture of HoVPNMPLS networkPE PESPEUPE UPECE CE CE CEVPN 1 VPN 1VPN 2 VPN 2Site 1 Site 2
