3Com S7906E Configuration Manual page 1530

Figure 1-13 Network diagram for nested VPN
Propagation of routing information
In a nested VPN network, routing information is propagated in the following process:
1) A provider PE and its CEs exchange VPNv4 routes, which carry information about users' internal
VPNs.
2) After receiving a VPNv4 route, a provider PE keeps the user's internal VPN information, and
appends the user's MPLS VPN attributes on the service provider network. That is, it replaces the
RD of the VPNv4 route with the RD of the user's MPLS VPN on the service provider network and
adds the export route-target (ERT) attribute of the user's MPLS VPN on the service provider
network to the extended community attribute list of the route. The internal VPN information of the
user is maintained on the provider PE.
3) The provider PE advertises VPNv4 routes which carry the comprehensive VPN information to the
other PEs of the service provider.
4) After another provider PE receives the VPNv4 routes, it matches the VPNv4 routes based on its
local VPNs. Each local VPN accepts routes of its own and advertises them to its connected
sub-VPN CEs (such as CE 3 and CE 4, or CE 5 and CE 6 in
provider PE through an IPv4 network, the PE advertises IPv4 routes to the CE; If a CE is connected
to a provider PE through a VPNv4 connection (a user MPLS VPN network), the PE advertises
VPNv4 routes to the CE.
Benefits
The nested VPN technology features the following main benefits:
Support for VPN aggregation. It can aggregate a user's internal VPNs into one VPN on the service
provider's MPLS VPN network.
Support for both symmetric networking and asymmetric networking. It allows sites of the same
VPN to have the same number or different numbers of internal VPNs.
Support for multiple levels of nesting of user internal VPNs.
Figure
1-15
1-13). If a CE is connected to a