3Com MSR 50 Series Configuration Manual page 1475

The hierarchical PE consists of multiple UPEs and SPEs, which function together as
a traditional PE.
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With the HoVPN solution, PE functions are implemented hierarchically. Hence, the
solution is also called hierarchy of PE (HoPE).
UPEs and SPEs play different roles:
A UPE allows user access. It maintains the routes of the VPN sites that are
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directly connected with it, It does not maintain the routes of the remote sites in
the VPN, or only maintains their summary routes. A UPE assigns inner labels to
the routes of its directly connected sites, and advertises the labels to the SPE
along with VPN routes through MP-BGP.
An SPE manages and advertises VPN routes. It maintains all the routes of the
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VPNs connected through UPEs, including the routes of both the local and
remote sites. An SPE does not advertise routes of the remote sites to UPEs; it
only advertises to UPEs the default routes of VPN instances or summary routes
along with labels.
Different roles mean different requirements:
SPE: An SPE is required to have large-capacity routing table, high forwarding
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performance, and fewer interface resources.
UPE: A UPE is required to have small-capacity routing table, low forwarding
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performance, but higher access capability.
HoVPN takes full use of both the high performance of SPEs and the high access
capability of UPEs.
Note that the concepts of SPE and UPE are relative. In the hierarchical PE
architecture, a PE may be the SPE of its underlayer PEs and a UPE of its SPE at the
same time.
The HoPE and common PEs can coexist in an MPLS network.
2 SPE-UPE
The MP-BGP running between SPE and UPE can be either MP-IBGP or MP-EBGP.
Which one to use depends on whether the UPE and SPE belong to a same AS.
With MP-IBGP, in order to advertise routes between IBGP peers, the SPE acts as the
RR and advertises routes from IBGP peer UPE to IBGP peer SPE. However, it does
not act as the RR of the other PEs.
3 Recursion and extension of HoVPN
HoVPN supports HoPE recursion:
A HoPE can act as a UPE to form a new HoPE with an SPE.
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A HoPE can act as an SPE to form a new HoPE with multiple UPEs.
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HoVPN supports multi-level recursion.
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With recursion of HoPEs, a VPN can be extended infinitely in theory.
MPLS L3VPN Overview 1475