3Com S7906E Configuration Manual page 1518

Sites connected to the same provider network can be classified into different sets by policies. Only the
sites in the same set can access each other through the provider network. Such a set is called a VPN.
Address space overlapping
Each VPN independently manages the addresses that it uses. The assembly of such addresses for a
VPN is called an address space.
The address spaces of VPNs may overlap. For example, if both VPN 1 and VPN 2 use the addresses
on network segment 10.110.10.0/24, address space overlapping occurs.
VPN instance
In MPLS VPN, routes of different VPNs are identified by VPN instance.
A PE creates and maintains a separate VPN instance for each VPN at a directly connected site. Each
VPN instance contains the VPN membership and routing rules of the corresponding site. If a user at a
site belongs to multiple VPNs at the same time, the VPN instance of the site contains information about
all the VPNs.
For independency and security of VPN data, each VPN instance on a PE maintains a relatively
independent routing table and a separate label forwarding information base (LFIB). VPN instance
information contains these items: the LFIB, IP routing table, interfaces bound to the VPN instance, and
administration information of the VPN instance. The administration information of the VPN instance
includes the route distinguisher (RD), route filtering policy, and member interface list.
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, as shown in
Figure 1-2 VPN-IPv4 address structure
Route distinguisher (8 bytes)
2 bytes
Type Administor subfield
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.
Figure 1-2.
6 bytes
Assigned number subfield
1-3
4 bytes
IPv4 address prefix