H3C Technologies H3C S7500E Series Switches - Manual (Page 89)

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z If LGP Shortcut is enabled only, only the router enabled with it can use LSPs for routing.
For configuration of this feature, see MPLS TE Configuration in the MPLS Configuration Guide.
VPN
OSPF supports multi-instance, which can run on PEs in VPN networks.
In BGP MPLS VPN networks, multiple sites in the same VPN can use OSPF as the internal routing
protocol, but they are treated as different ASs. An OSPF route learned by a site will be forwarded to
another site as an external route, which leads to heavy OSPF routing traffic and management issues.
Configuring area IDs on PEs can differentiate VPNs. Sites in the same VPN are considered as directly
connected. PE routers then exchange OSPF routing information like on a dedicated line; thus network
management and OSPF operation efficiency are improved.
For configuration of this feature, see MPLS L3VPN Configuration in the MPLS Configuration Guide.
OSPF sham link
An OSPF sham link is a point-to-point link between two PE routers on the MPLS VPN backbone.
In general, BGP peers exchange routing information on the MPLS VPN backbone using the BGP
extended community attribute. OSPF running on a PE at the other end utilizes this information to
originate a Type-3 summary LSA as an inter-area route between the PE and CE.
If a router connects to a PE router in the same area and establishes an internal route (backdoor route)
to a destination, in this case, since an OSPF intraarea route has a higher priority than a backbone
route, VPN traffic will always travel on the backdoor route rather than the backbone route. To avoid this,
an unnumbered sham link can be configured between PE routers, connecting the router to another PE
router via an intraarea route with a lower cost.
For sham link configuration, see MPLS L3VPN Configuration in the MPLS Configuration Guide.
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