H3C Technologies H3C S7500E Series Switches - Manual (Page 229)

6-4
You are recommended to configure a distinct RD for each VPN instance on a PE, guaranteeing that
routes to the same CE use the same RD. The VPN-IPv4 address with an RD of 0 is in fact a globally
unique IPv4 address.
By prefixing a distinct RD to a specific IPv4 address prefix, you get a globally unique VPN IPv4
address prefix.
An RD can be related to an autonomous system (AS) number, in which case it is the combination of
the AS number and a discretionary number; or be related to an IP address, in which case it is the
combination of the IP address and a discretionary number.
An RD can be in one of the following three formats distinguished by the Type field:
z
When the value of the Type field is 0, the Administrator subfield occupies two bytes, the Assigned
number subfield occupies four bytes, and the RD format is 16-bit AS number:32-bit user-defined
number . For example, 100:1.
z
When the value of 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 . For example, 172.1.1.1:1.
z
When the value of 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.
For the global uniqueness of an RD, you are not recommended to set the Administrator subfield to any
private AS number or private IP address.
VPN target attributes
MPLS L3VPN uses the BGP extended community attributes called VPN target attributes, or route
target attributes, to control the advertisement of VPN routing information.
A VPN instance on a PE supports two types of VPN target attributes:
z
Export target attribute: A local PE sets this type of VPN target attribute for VPN-IPv4 routes learnt
from directly connected sites before advertising them to other PEs.
z
Import target attribute: A PE checks the export target attribute of VPN-IPv4 routes advertised by
other PEs. If the export target attribute matches the import target attribute of the VPN instance,
the PE adds the routes to the VPN routing table.
In other words, VPN target attributes define which sites can receive VPN-IPv4 routes, and from which
sites that a PE can receive routes.
Like RDs, VPN target attributes can be of three formats:
z 16-bit AS number :32-bit user-defined number. For example, 100:1.
z 32-bit IPv4 address :16-bit user-defined number. For example, 172.1.1.1:1.
z 32-bit AS number :16-bit user-defined number, where the minimum value of the AS number is
65536. For example, 65536:1.
MP-BGP
Multiprotocol extensions for BGP-4 (MP-BGP) advertises VPN composition information and routes
between PEs. It is backward compatible and supports both traditional IPv4 address family and other
address families, such as VPN-IPv4 address family.
Using MP-BGP can guarantee that private routes of a VPN are advertised only in the VPN and
implement communications between MPLS VPN members.