175
HoVPN divides PEs into underlayer PEs (UPEs) or user-end PEs, and superstratum PEs (SPEs) or service
provider-end PEs. UPEs and SPEs have different functions and comprise a hierarchical PE. The HoPE and
common PEs can coexist in an MPLS network.
Figure 53
Basic architecture of HoVPN
As shown in
, UPEs and SPEs play the following different roles:
•
A UPE is directly connected to CEs. It provides user access. It maintains the routes of directly
connected VPN sites. It does not maintain the routes of the remote sites in the VPN, or it only
maintains their summary routes. A UPE assigns inner labels to the routes of its directly connected
sites, and advertises the labels along with VPN routes to the SPE through MP-BGP. A UPE features
high access capability, small routing table capacity, and low forwarding performance.
•
An SPE is connected to UPEs and is in the internal network. It manages and advertises VPN routes.
It maintains all the routes of the VPNs connected through UPEs, including the routes of both the local
and remote sites. An SPE advertises routes along with labels to UPEs, including the default routes of
VPN instances or summary routes and the routes permitted by the routing policy. By using routing
policies, you can control which sites in a VPN can communicate with each other. An SPE features
large routing table capacity, high forwarding performance, and fewer interface resources.
Either MP-IBGP or MP-EBGP can run between SPE and UPE. When MP-IBGP runs between SPE and UPEs,
the SPE acts as the RR of multiple UPEs to reflect routes between UPEs.
HoVPN supports HoPE recursion:
•
An HoPE can act as a UPE to form a new HoPE with an SPE.
•
An HoPE can act as an SPE to form a new HoPE with multiple UPEs.
HoVPN supports multilevel recursion. In HoPE recursion, the concepts of SPE and UPE are relative. A PE
might be the SPE of its underlayer PEs and a UPE of its SPE at the same time.
