H3C Technologies H3C S12500-X Series Switches - Manual (Page 126)

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MP-BGP advertises all the VPN routes of UPEs to the SPEs, and advertises the default routes of the VPN
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. Each UPE maintains only VPN routes of its
directly connected sites. An MPE has fewer routes than the SPE but has more routes than a UPE.
OSPF VPN extension
This section describes the OSPF VPN extension. For more information about OSPF, see Layer 3—IP
Routing Configuration Guide.
OSPF for VPNs on a PE
OSPF is a commonly used IGP protocol. Running OSPF between a PE and a CE can simplify CE
configuration and management because the CEs only need to support OSPF. In addition, if the customers
require MPLS L3VPN services through a conventional OSPF backbone, using OSPF between a PE and a
CE can simplify the transition.
For OSPF to run between CE and PE, the PE must support multiple OSPF processes. Each OSPF process
corresponds to a VPN instance and maintains its own interfaces and routing table.
The following describes OSPF configurations between a PE and a CE:
• OSPF area configuration between a PE and a CE:
The 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 because
OSPF requires that the backbone area be contiguous.
If a VPN site contains an OSPF area 0, the PE must be connected to the backbone area of the VPN
site through area 0. You can configure a virtual link to connect the CE to the PE.
• BGP/OSPF interaction:
PEs advertise VPN routes to each other through BGP and to CEs through OSPF.
Conventional OSPF considers that two sites are in different ASs even if they belong to the same
VPN. Therefore, the routes that one site learns are advertised to the other as external routes. This
results in OSPF traffic and network management problems.
Extended OSPF supports multiple instances to address OSPF traffic and network management
problems. When configured correctly, OSPF sites are considered directly connected, and PEs
exchange OSPF routing information as they do on a dedicated line. This simplifies network
management and makes OSPF applications more effective.
As shown in
Figure 44 , PE 1 and PE 2 are connected through the MPLS backbone. CE 11, CE 21,
and CE 22 belong to VPN 1. Assume that CE 11, CE 21, and CE 22 belong to the same OSPF
domain. PEs advertise VPN 1 routes by using the following process:
a. PE 1 redistributes OSPF routes of CE 11 into BGP.
b. PE 1 advertises the VPN routes to PE 2 through BGP.
c. PE 2 redistributes the BGP VPN routes into OSPF and advertises them to CE 21 and CE 22.