3Com MSR 50 Series Configuration Manual page 1474

1474 C 79: MPLS L3VPN C
HAPTER ONFIGURATION
Most of the current network schemes use the typical hierarchical architecture. For
example, the MAN architecture contains typically three layers, namely, the core
layer, convergence layer, and access layer. From the core layer to the access layer,
the performance requirements on the devices reduce while the network 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 network is influenced.
Due to the above difference, you are faced with the scalability problem when
deploying PEs at any of the three layers. Therefore, the plane model is not
applicable to the large-scale VPN deployment.
2 HoVPN
To solve the scalability problem of the plane model, MPLS L3VPN must transition
to the hierarchical model.
In MPLS L3VPN, hierarchy of VPN (HoVPN) was proposed to meet that
requirement. With HoVPN, the PE functions can be distributed among multiple
PEs, which take different roles for the same functions and form a hierarchical
architecture.
As in the typical hierarchical network model, HoVPN has different requirements on
the devices at different layers of the hierarchy.
Implementation of HoVPN
1 Basic architecture of HoVPN
Figure 417 Basic architecture of HoVPN
UPE
VPN 1
Site 1
CE
As shown in Figure 417, devices directly connected to CEs are called underlayer
PEs (UPEs) or user-end PEs, whereas devices that are connected with UPEs and are
in the internal network are called superstratum PEs (SPE) or service provider-end
PEs.
PE PE
MPLS network
SPE
VPN 2 VPN 1
CE CE
UPE
VPN 2
Site 2
CE