244Figure 72 Recursion of HoPEsFigure 72 shows a three-level HoPE. The PE in the middle is called the middle-level PE (MPE). MP-BGPruns between SPE and MPE, and between MPE and UPE.The term "MPE" does not really exist in a HoVPN model. It is used here just for the convenience ofdescription.MP-BGP advertises all the VPN routes of the UPEs to the SPEs, and advertises the default routes of theVPN instance of the SPEs or the VPN routes permitted by the routing policies to the UPEs.The SPE maintains the VPN routes of all sites in the HoVPN, while each UPE maintains only VPN routesof its directly connected sites. The number of routes maintained by the MPE is between the two.OSPF VPN extensionThis section focuses on the OSPF VPN extension. For more information about OSPF, see Layer 3—IPRouting Configuration Guide.OSPF for VPNs on a PEOSPF is a prevalent IGP protocol. It often runs between PE and CE to simplify CE configuration andmanagement because the CEs only need to support OSPF. In addition, if the customers require MPLSL3VPN services through conventional OSPF backbone, using OSPF between PE and CE can simplify thetransition.For OSPF to run between CE and PE, the PE must support multiple OSPF processes. Each OSPF processmust correspond to a VPN instance and have its own interface and routing table.The following describes details of OSPF configuration between PE and CE.• Configuration of OSPF areas between PE and CEThe OSPF area between a PE and a CE can be either a non-backbone area or a backbone area.In the OSPF VPN extension application, the MPLS VPN backbone is considered the backbone area(area 0). The area 0 of each VPN site must be connected to the MPLS VPN backbone becauseOSPF requires that the backbone area be contiguous.If a VPN site contains an OSPF area 0, the connected PE must be connected to the backbone areaof the VPN site through area 0. You can configure a logical connection by using a virtual link.• BGP/OSPF interactionPEs advertise VPN routes to each other through BGP and to CEs through OSPF.SPEMPEUPEUPEUPECE CE CE CE
