3Com 3C13636 Configuration Manual page 873

Router 3000 ethernet family

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3Com Router 3000 Ethernet Family

Configuration Guide

OSPF 100 VPN1

OSPF 200 VPN2

Figure 3-6 Application of OSPF in VPN

The standard BGP/OSPF interaction enables PE2 to advertise the BGP VPN routes to

CE21 and CE22 through Type5 LSAs (ASE LSAs). However, CE11, CE21, and CE22

belong to the same OSPF area, and the route advertisement between them depends

on Type3 LSAs (inter-domain routes).

To avoid the above cases, the PE applies a modified BGP/OSPF interaction process

(shorted as BGP/OSPF interoperability). It can advertise the routes from a Site to

another, and differentiate the routes from real AS-External routes. The process requires

the extension community attribute of BGP and the information identifying OSPF

The V 2.41 requires that each OSPF domain have a Domain ID. It is recommended to

configure a same Domain ID for all OSPF instances in the network related to each VPN

instance, or adopt the default ID 0. In this case, all the VPN routes with the same

Domain ID come from the same VPN instance.

Routing loop detection

Suppose PE connects to CE through the OSPF backbone area, and a VPN Site

connects to multiple PEs. When a PE advertises BGP VPN routes learnt from

MPLS/BGP to a VPN Site through LSAs, the LSAs may possibly be received by

another PE, thus resulting in the routing loop.

To avoid the routing loop, the PE sets the flag bit DN for BGP VPN routes learnt from

MPLS/BGP when originating Type3 LSAs, regardless of whether PE connects to CE

through the OSPF backbone area. The PE ignores the Type3 LSAs for DN settings

during the route calculation of OSPF process.

To advertise a route from another OSPF domain to CE, the PE should claim an ASBR

and advertise the route as Type5 LSA. The VPN Route Tag configured for an OSPF

instance is contained in Type5 or Type7 LSAs. The PE ignores the Type5 or Type7 LSA

3Com Corporation

Chapter 3 BGP/MPLS VPN Configuration

OSPF 100 VPN1

OSPF 200 VPN1

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3c13636-us - router 3036 3000 series