4An RD can be in one of the following formats distinguished by the Type field:• When the value of the Type field is 0, the Administrator subfield occupies two bytes, the Assignednumber subfield occupies four bytes, and the RD format is 16-bit AS number:32-bit user-definednumber. For example, 100:1.• When the value of the Type field is 1, the Administrator subfield occupies four bytes, the Assignednumber subfield occupies two bytes, and the RD format is 32-bit IPv4 address:16-bit user-definednumber. For example, 172.1.1.1:1.• When the value of the Type field is 2, the Administrator subfield occupies four bytes, the Assignednumber subfield occupies two bytes, and the RD format is 32-bit AS number:16-bit user-definednumber, where the minimum value of the AS number is 65536. For example, 65536:1.To guarantee global uniqueness for an RD, do not set the Administrator subfield to any private ASnumber or private IP address.Route target attributesMPLS L3VPN uses the BGP extended community attributes called "route target" attributes to control theadvertisement of VPN routing information.A VPN instance on a PE supports the following types of route target attributes:• Export target attribute: A local PE sets this type of route target attribute for VPN-IPv4 routes learnedfrom directly connected sites before advertising them to other PEs.• Import target attribute: A PE checks the export target attribute of VPN-IPv4 routes advertised byother PEs. If the export target attribute matches the import target attribute of the VPN instance, thePE adds the routes to the VPN routing table.In other words, route target attributes define which sites can receive VPN-IPv4 routes, and from whichsites that a PE can receive routes.Similar to RDs, route target attributes can be of the following formats:• 16-bit AS number:32-bit user-defined number. For example, 100:1.• 32-bit IPv4 address:16-bit user-defined number. For example, 172.1.1.1:1.• 32-bit AS number:16-bit user-defined number, where the minimum value of the AS number is 65536.For example, 65536:1.Multi-VPN-instance CEUsing tunnels, MPLS L3VPN implements private network data transmission over the public network.However, the traditional MPLS L3VPN architecture requires each VPN instance exclusively use a CE toconnect with a PE, as shown in Figure 1.For better services and higher security, a private network is usually divided into multiple VPNs to isolateservices. To meet these requirements, you can configure a CE for each VPN, which increases users’device expenses and maintenance costs. Or, you can configure multiple VPNs to use the same CE andthe same routing table, which sacrifices data security.Using the Multi-VPN-Instance CE (MCE) function of the switch, you can remove the contradiction of lowcost and high security in multi-VPN networks. With MCE configured, a CE can bind each VPN in anetwork with a VLAN interface on the CE, and create and maintain a separate routing table (multi-VRF)for each VPN. This separates the forwarding paths of packets of different VPNs and, in conjunction withthe PE, can correctly advertise the routes of each VPN to the peer PE, ensuring the normal transmissionof VPN packets over the public network.
