H3C Technologies H3C S10500 Series Switches - Manual (Page 234)

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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 segment 10.110.10.0/24 and each advertise a route to the
segment, 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.
Figure 54 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 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 an RD, 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 the following types of VPN target attributes: