3Com MSR 50 Series, Configuration Manual

Page: 1462 / 2443

1462 C
HAPTER
79: MPLS L3VPN C
ONFIGURATION
Figure 406 VPN-IPv4 address structure
When a PE receives an ordinary IPv4 route from a CE, it must advertise the VPN
route to the peer PE. The uniqueness of a VPN route is implemented by adding an
RD to the route.
A service provider can independently assign RDs provided the assigned RDs are
unique. Thus, a PE can advertise different routes to VPNs even if the VPNs are from
different service providers and are using the same IPv4 address space.
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 either of the following two 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.
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
Administor subfield Assigned number subfield Type
2 bytes 4 bytes
IPv4 address prefix
6 bytes
Route distinguisher (8 bytes)