1-43z If you start OSPF but do not configure the router ID, the system will automatically elect one.However, the same election rules produce the same router ID. Therefore, you are recommended toconfigure the router ID when starting an OSPF process. For the election rules, refer to OSPFConfiguration in the IP Routing Volume.z If you configure multiple OSPF VPN instances but do not configure the route tag, the system willautomatically create one based on the AS number configured. If you do not configure BGP, the tagwill be 0. However, the same calculation rule produces the same tag, and hence the same tag willbe created for multiple OSPF VPN instances on the same PE or PEs with the same AS number.Therefore, you are recommended to configure different tags for different OSPF VPN instance.Configuring BGP AS Number SubstitutionConfiguration PrerequisitesBefore configuring BGP AS number substitution, complete these tasks:z Configuring basic MPLS L3VPNz Configuring CEs at different sites to have the same AS numberConfiguration ProcedureWhen CEs at different sites have the same AS number, you need to configure the BGP AS numbersubstitution function to avoid route loss.With the BGP AS number substitution function, when a PE advertises a route to a CE of the specifiedpeer, if an AS number identical to that of the CE exist in the AS_PATH of the route, it will be replacedwith that of the PE before the route is advertised.Follow these steps to configure the BGP AS number substitution function:To do… Use the command… RemarksEnter system view system-view —Enter BGP view bgp as-number RequiredEnter BGP VPN instance view ipv4-family vpn-instancevpn-instance-name RequiredEnable the BGP AS numbersubstitution functionpeer { ip-address |group-name } substitute-asRequiredDisabled by defaultFor information about the peer { ip-address | group-name } substitute-as command, refer to BGPCommands in the IP Routing Volume.
