3com S7906E Installation Instruction

 

1-3 

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 in 

network segment 10.110.10.0/24, address space overlapping occurs. 

In MPLS VPN, route separation between VPNs is implemented by VPN instance. 

A PE creates and maintains a separate VPN instance for each 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. 

 

LFIBs of VPN instances exist on only PEs supporting MPLS. No LFIBs of VPN instances exist on 

MCE-capable devices. 

 

Traditional BGP cannot process VPN routes which have overlapping address spaces. If, for example, 

both VPN 1 and VPN 2 use addresses in the segment 10.110.10.0/24 and 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 Structure of a VPN-IPv4 address 

Administor subfield     Assigned number subfield

Type

2 bytes

4 bytes

IPv4 address  prefix

6 bytes

Route Distinguisher (8 bytes)

 

 

When a PE receives an ordinary IPv4 route from a CE, it must redistribute 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. In this way, 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.