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A CE is usually a router. After a CE establishes adjacency with a directly connected PE, it advertises its
VPN routes to the PE and learns remote VPN routes from the PE. A CE and a PE use BGP/IGP to
exchange routing information. You can also configure static routes between them.
After a PE learns the VPN routing information of a CE, it uses BGP to exchange VPN routing information
with other PEs. A PE maintains routing information about only VPNs that are directly connected, rather
than all VPN routing information on the provider network.
A P router maintains only routes to PEs. It does not need to know anything about VPN routing information.
When VPN traffic travels over the MPLS backbone, the ingress PE functions as the ingress LSR, the egress
PE functions as the egress LSR, while P routers function as the transit LSRs.
MPLS L3VPN concepts
Site
Site is often mentioned in the VPN. Its meanings are described as follows:
•
A site is a group of IP systems with IP connectivity that does not rely on any service provider network
to implement.
•
The classification of a site depends on the topology relationship of the devices, 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.
•
A site is connected to a provider network through one or more CEs. A site can 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.
