3
Figure 2 VPN-IPv4 address structure
As shown in Figure 2, a VPN-IPv4 address consists of 12 bytes. The first eight bytes represent the RD,
followed by a four-byte IPv4 prefix. The RD and the IPv4 prefix form a unique VPN-IPv4 prefix.
An RD can be in one of the following formats:
•When the Type field is 0, the Administrator subfield occupies two bytes, the Assigned number
subfield occupies four bytes, and the RD format is 16 - b i t A S n u m b e r :32-bit user-defined number. For
example, 100:1.
•When the Type field is 1, the Administrator subfield occupies four bytes, the Assigned number
subfield occupies two bytes, and the RD format is 32-bit IPv4 address:16-bit user-defined number.
Fo r e xa m p l e , 172.1.1.1:1.
•When the Type field is 2, the Administrator subfield occupies four bytes, the Assigned number
subfield occupies two bytes, and the RD format is 32-bit AS number:16-bit user-defined number,
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 AS
number or private IP address.
Route target attribute
MPLS L3VPN uses route target community attributes to control the advertisement of VPN routing
information. A VPN instance on a PE supports the following types of route target attributes:
•Export target attribute—A PE sets the export target attribute for VPN-IPv4 routes learned from
directly connected sites before advertising them to other PEs.
•Import target attribute—A PE checks the export target attribute of VPN-IPv4 routes received from
other PEs. If the export target attribute matches the import target attribute of a VPN instance, the PE
adds the routes to the routing table of the VPN instance.
Route target attributes define which sites can receive VPN-IPv4 routes, and from which sites a PE can
receive routes.
Like RDs, route target attributes can be one of the following formats:
•16 - b i t A S n u m b e r :32-bit user-defined number. For example, 100:1.
•32-bit IPv4 address:16-bit user-defined number. F or e xa m p l e , 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.
MCE overview
BGP/MPLS VPN transmits private network data through MPLS tunnels over the public network. However,
the traditional MPLS L3VPN architecture requires that each VPN instance use an exclusive CE to connect
to a PE, as shown in Figure 1.
A private network is typically divided into multiple VPNs to isolate services. To meet these requirements,
you can configure a CE for each VPN, which increases device expense and maintenance costs. Or, you
Type
2 bytes 4 bytes
IPv4 address prefix
6 bytes
Route Distinguisher (8 bytes)
Assigned number subfieldAdministrator subfield
