3Com MSR 50 Series Configuration Manual page 1461

A site is a group of IP systems with IP connectivity that does not rely on any
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service provider network to implement.
The classification of a site depends on the topology relationship of the devices,
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rather than the geographical positions, though the devices at a site are
adjacent to each other geographically in most cases.
The devices at a site can belong to multiple VPNs.
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A site is connected to a provider network through one or more CEs. A site can
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contain many CEs, but a CE can belong to only one site.
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 advertises a route to the segment, BGP selects only one
of them, which results in loss of the other route.
PEs use "MP-BGP" on page 1463
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
MPLS L3VPN Overview
to advertise VPN routes, and use VPN-IPv4
Figure
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406.