H3C Technologies H3C S12500-X Series Switches - Manual (Page 127)

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Figure 44 Application of OSPF in VPN
With the standard BGP/OSPF interaction, PE 2 advertises the BGP VPN routes to CE 21 and CE
22 in Type 5 LSAs (ASE LSAs). However, CE 11, CE 21, and CE 22 belong to the same OSPF
domain, and route advertisements between them should use Type 3 LSAs (inter-area routes).
With the extended BGP/OSPF interaction, PEs advertise routes from one site to another site in
Type 3 LSAs. The process requires that extended BGP community attributes include information for
identifying the OSPF attributes.
Each OSPF domain must have a domain ID. H3C recommends that you configure the same
domain ID or adopt the default ID for all OSPF processes of the same VPN, so the system can know
that VPN routes with the same domain ID are from the same VPN.
• Routing loop detection:
If a CE and a PE are connected through the OSPF backbone area, when a PE advertises BGP VPN
routes learned from MPLS/BGP to the VPN site through LSAs, the LSAs might be received by
another PE, resulting in a routing loop.
To avoid routing loops, when creating Type 3 LSAs, the PE always sets the flag bit DN for BGP
VPN routes learned from MPLS/BGP, regardless of whether the PE and the CE are connected
through the OSPF backbone. When performing route calculation, the OSPF process of the PE
ignores the Type 3 LSAs whose DN bit is set.
If the PE needs to advertise routes from other OSPF domains to a CE, it must indicate that it is the
ASBR, and advertise the routes in Type 5 LSAs.
OSPF sham link
On a PE, BGP routes received from the peer PE are redistributed into OSPF, and OSPF advertises these
routes in Type 3 summary LSAs (inter-area routes) to the CE. As shown in Figure 45 , both site 1 and site
2 belong to VPN 1 and OSPF area 1. Both an intra-area route (called a backdoor link) and an inter-area
route exist between the two sites. The inter-area route is not preferred by OSPF because its priority is
lower than the intra-area route priority.