H3C Technologies H3C S7500E Series Switches - Manual (Page 244)

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OSPF VPN Extension
This section focuses on the OSPF VPN extension. For more information about OSPF, see OSPF
Configuration in the Layer 3 - IP Routing Configuration Guide.
OSPF multi-instance on PE
OSPF is a prevalent IGP protocol. In many cases, VPN clients are connected through BGP peers, and
the clients often run OSPF. Running OSPF between PEs and CEs can simplify the configuration and
management of the CEs, because the CEs only need to support OSPF. In addition, if the customers
require MPLS L3VPN services through conventional OSPF backbone, using OSPF between PEs and
CEs can simplify the transition.
For OSPF to run between CEs and PEs, the PEs must support multiple OSPF instances. Each OSPF
instance must correspond to a VPN instance and have its own interface and routing table.
The following describes details of OSPF configuration between PEs and CEs.
1) Configuration of OSPF areas between PEs and CEs
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). Since OSPF requires that the backbone area must be contiguous, the area 0 of each VPN
site must be connected with the MPLS VPN backbone.
That is, if a VPN site contains an OSPF area 0, the PE connected with the CE must be connected with
the area 0 in this VPN site through an area 0 (the virtual link can be used for logical connection).
2) BGP/OSPF interaction
With OSPF running between PEs and CEs, PEs advertise VPN routes to each other through BGP and
to CEs through OSPF.
With conventional OSPF, two sites are considered to be 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 higher OSPF traffic and network management problems that should have been avoided
otherwise.
Currently, OSPF supports multiple instances and therefore can address the above problems. Properly
configured, OSPF sites are considered directly connected, and PEs can exchange OSPF routing
information as they are using dedicated lines. This improves the network management and makes
OSPF applications more effective.
As shown in Figure 6-16 , PE 1 and PE 2 are connected through the MPLS backbone, while CE 11, CE
21, and CE 22 belong to VPN 1. Assumes that all the routers in the figure belong to the same AS, that
is, CE 11, CE 21, and CE 22 belong to the same OSPF domain. The advertisement procedure of VPN
1 routes is as follows:
z
At first, PE 1 redistributes OSPF routes from CE 11 into BGP.
z
Then, PE 1 advertises the VPN routes to PE 2 through BGP.