HP 12500 Series Configuration Manual page 201

VPN-IPv4 address
Traditional BGP cannot process VPN routes which have overlapping address spaces. If, for example,
both VPN 1 and VPN 2 use addresses on the subnet 10.1 10.10.0/24 and each advertise a route to the
subnet, BGP selects only one of them, which results in loss of the other route.
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 48 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. So, 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.
HP recommends that you 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 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, do not set the Administrator subfield to any private AS number or
private IP address.
BGP extended community attirubtes
• 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.
Figure 48.
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