3Com 8807 Configuration Manual page 535

Hierarchical BGP/MPLS
VPN Implementation
Configuring basic MPLS capability on the MPLS backbone network.
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Configuring MPLS LDP and setting up LDP LSP on the MPLS backbone network.
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Configuring BGP on the MPLS backbone network (create IBGP peers between
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provider PEs).
Configuring basic MPLS capability on user-end network (including customer
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PEs).
As PE is required to aggregate multiple VPN routes on a BGP/MPLS VPN, it is prone
to forming a bottleneck in a large-scale deployment or in the case that PE capacity
is small. To solve the problem, 3Com Corporation introduced the HoVPN
(Hierarchy of VPN, Hierarchical BGP/MPLS VPN) solution.
Hierarchical BGP/MPLS VPN divides an MPLS VPN into several MPLS VPNs in a
hierarchical network structure. Each VPN takes on a role depending on its level.
There are high performance requirements in routing and forwarding on the PEs at
the higher level of MPLS VPN, because they are primarily used for connecting the
backbone networks and providing access service for huge VPN clients. However,
such requirements are relatively low for PEs at the lower level of the network as
they primarily function to access the VPN clients at the edges. Congruous with the
IP network model, HoVPN model improves the scalability of BGP/MPLS VPN, and
hence allows lower-layer MPLS VPNs comprising low-end equipment to provide
MPLS VPN accessing and interconnect through the high-end MPLS VPN backbone.
As shown in Figure 131, the PEs directly connected with user devices are called
UPE (underlayer PE or user-end PE); the devices in the core network connected
with the UPEs are called SPE (superstratum PE or service-provider-end PE).
Hierarchical PEs have the same appearance as that of the traditional PEs and can
coexist with other PEs in the same MPLS network.
UPEs are responsible for user access; they only maintain the routes of directly
connected VPN sites, but not that of the remote sites. SPEs, however, are
responsible for the maintenance and advertisement of VPN routes; they maintain
all the routes of the VPNs connected by their UPEs, including the routes in both
local and remote sites.
UPE and SPE are relative concepts. In a multi-layer PE architecture, an upper layer
PE is an SPE for its lower layer PE, and a lower layer PE is an UPE for its upper layer
PE.
The MBGP runs between SPE and UPE can be either MP-IBGP or MP-EBGP,
depending on whether the SPE and the UPE are in the same AS.
BGP/MPLS VPN Overview 535