H3C Technologies H3C SR8800 - Manual (Page 235)

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PEs use MP-BGP to advertise VPN routes, and use VPN-IPv4 address family to solve the problem with
traditional BGP.
A VPN-IPv4 address consists of 12 bytes. The first eight bytes represent the RD, followed by a 4-byte IPv4
address prefix, as shown in Figure 56 .
Figure 56 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.
Configure a distinct RD for each VPN instance on a PE, so 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 it can 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.
To guarantee global uniqueness for RDs, do not 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 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 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.