3Com S7906E Configuration Manual page 1519

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:
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.
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.
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:
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.
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:
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.
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.
Routing policy
In addition to the import and export extended communities for controlling VPN route advertisement, you
can also configure import and export routing policies to control the injection and advertisement of VPN
routes more precisely.
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