3Com Switch 8800 Advanced Software V5 Configuration Guide

OSPF Configuration Task List 321
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.
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 Type3 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) for a special 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 low cost.
Protocols and Standards ■ RFC 1765:OSPF Database Overflow
■ RFC 2328: OSPF Version 2
■ RFC 3101: OSPF Not-So-Stubby Area (NSSA) Option
■ RFC 3137: OSPF Stub Router Advertisement
■ RFC 3630: Traffic Engineering Extensions to OSPF Version 2
OSPF Configuration
Task List
To configure OSPF, perform the tasks described in the following sections:
Task Description
“Configuring OSPF Basic Functions” on page 323 Required
“Configuring OSPF Area Parameters” on page 324 Optional
“Configuring OSPF Network
Types” on page 325
“Configuring the OSPF
Network Type for an
Interface” on page 326
Optional
“Configuring an NBMA
Neighbor” on page 326
Optional
“Configuring a DR Priority for
an OSPF Interface” on page
326
Optional