1-22Figure 1-18 Application of BGP AS number substitutionIn Figure 1-18, both CE 1 and CE 2 use the AS number of 800. AS number substitution is enabled onPE 2 for CE 2. Before advertising updates received from CE 1 to CE 2, PE 2 finds that an AS number inthe AS_PATH is the same as that of CE 2 and hence substitutes its own AS number 100 for the ASnumber. In this way, CE 2 can normally receive the routing information from CE 1.AS number substitution also applies to a PE connecting multiple CEs through different interfaces, suchas PE 2 in Figure 1-18, which connects CE 2 and CE 3.For a multi-homed CE, that is, a CE connected with multiple PEs, the BGP AS number substitutionfunction must be used in combination with the site-of-origin (SOO) function. Otherwise, routing loopsmay appear.MPLS L3VPN Configuration Task ListComplete the following tasks to confTask RemarksConfiguring VPN Instances RequiredConfiguring Basic MPLS L3VPN RequiredConfiguring Inter-Provider VPN OptionalConfigure it as neededConfiguring Nested VPN OptionalConfigure it as neededConfiguring HoVPN OptionalConfigure it as neededConfiguring OSPF Sham Link OptionalConfigure it as needed
