3Com MSR 50 Series Configuration Manual page 1477

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 419, 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:
At first, PE 1 redistributes OSPF routes from CE 11 into BGP.
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Then, PE 1 advertises the VPN routes to PE 2 through BGP.
■
Finally, PE 2 redistributes the BGP VPN routes into OSPF and advertises them to
■
CE 21 and CE 22.
Figure 419 Application of OSPF in VPN
VPN 1
OSPF Area 0
Site 1
CE 11
CE 12
VPN 2
OSPF Area 1
Site 1 Site 1
With the standard BGP/OSPF interaction, PE 2 advertises the BGP VPN routes to CE
21 and CE 22 through Type 5 LSAs (ASE LSAs). However, CE 11, CE 21, and CE 22
PE 1
MPLS backbone
MPLS L3VPN Overview
VPN 1
OSPF Area 1
Site 2
CE 21
PE 2
CE 22
VPN 1
OSPF Area 2
Site 3
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