HP FlexNetwork 10500 Series Configuration Manual page 205

In the OSPF VPN extension application, the MPLS VPN backbone is considered the backbone area
(area 0). The area 0 of each site must be connected to the MPLS VPN backbone (physically
connected or logically connected through a virtual link) because OSPF requires that the backbone
area be contiguous.
BGP/OSPF interaction
If OSPF runs between PEs and CEs, each PE redistributes BGP routes to OSPF and advertises the
routes to CEs through OSPF. OSPF considers the routes redistributed from BGP as external routes
but the OSPF routes actually belong to the same OSPF domain. This problem can be resolved by
configuring the same domain ID for sites in an OSPF domain.
Figure 56 Network diagram for BGP/OSPF interaction
VPN 1
OSPF Area 0
Site 1
VPN 2
OSPF Area 1
Site 1 Site 1
As shown in
domain.
Before domain ID configuration, VPN 1 routes are advertised from CE 11 to CE 21 and CE 22 by
using the following process:
1. PE 1 redistributes OSPF routes from CE 11 into BGP, and advertises the VPN routes to PE 2
through BGP.
2. PE 2 redistributes the BGP routes to OSPF, and advertises them to CE 21 and CE 22 in AS
External LSAs (Type 5) or NSSA External LSAs (Type 7).
After domain ID configuration, VPN 1 routes are advertised from CE 11 to CE 21 and CE 22 by using
the following process:
3. PE 1 redistributes OSPF routes into BGP, adds the domain ID to the redistributed BGP VPNv4
routes as a BGP extended community attribute, and advertises the routes to PE 2.
4. PE 2 compares the domain ID in the received routes with the locally configured domain ID. If
they are the same and the received routes are intra-area or inter-area routes, OSPF advertises
these routes in Network Summary LSAs (Type 3). Otherwise, OSPF advertises these routes in
AS External LSAs (Type 5) or NSSA External LSAs (Type 7).
CE 11
PE 1
CE 12
Figure 56, CE 11, CE 21, and CE 22 belong to the same VPN and the same OSPF
PE 2
MPLS backbone
196
VPN 1
OSPF Area 1
Site 2
CE 21
CE 22
VPN 1
OSPF Area 2
Site 3