192
Figure 56 Network diagram for nested VPN
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Propagation of routing information
In a nested VPN network, routing information is propagated by using the following process:
1.
After receiving VPN routes from customer CEs, a customer PE advertises VPN-IPv4 routes to
the provider CEs through MP-BGP.
2.
The provider CEs advertise the VPN-IPv4 routes to a provider PE through MP-BGP.
3.
After receiving a VPN-IPv4 route, the provider PE keeps the customer's internal VPN
information, and appends the customer's MPLS VPN attributes on the service provider network.
It replaces the RD of the VPN-IPv4 route with the RD of the customer's MPLS VPN on the
service provider network. It also adds the export route-target (ERT) attribute of the customer's
MPLS VPN on the service provider network to the extended community attribute list of the route.
The internal VPN information for the customer is maintained on the provider PE.
4.
The provider PE advertises VPN-IPv4 routes carrying the comprehensive VPN information to
the other PEs of the service provider.
5.
After another provider PE receives the VPN-IPv4 routes, it matches the VPN-IPv4 routes to the
import targets of its local VPNs. Each local VPN accepts routes of its own and advertises them
to provider CEs. If a provider CE (such as CE 7 and CE 8 in
772H
Figure 56
) is connected to a
provider PE through an IPv4 connection, the PE advertises IPv4 routes to the CE. If it is a
VPN-IPv4 connection (a customer MPLS VPN network), the PE advertises VPN-IPv4 routes to
the CE.
6.
After receiving VPN-IPv4 routes from the provider CE, a customer PE matches those routes to
local import targets. Each customer VPN accepts only its own routes and advertises them to
connected customer CEs (such as CE 3, CE 4, CE 5, and CE 6 in
773H
Figure 56
).
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HoVPN
Hierarchy of VPN (HoVPN), also called Hierarchy of PE (HoPE), prevents PEs from being
bottlenecks and is applicable to large-scale VPN deployment.
HoVPN divides PEs into underlayer PEs (UPEs) or user-end PEs, and superstratum PEs (SPEs) or
service provider-end PEs. UPEs and SPEs have different functions and comprise a hierarchical PE.
The HoPE and common PEs can coexist in an MPLS network.
Provider PE
CE 3
CE 4
Provider MPLS
VPN backbone
P
VPN A-1
VPN A-1
VPN A-2
CE 7
Provider PE
VPN A-2
CE 8
Customer MPLS
VPN
Customer PE
CE 5
CE 6
VPN A-1
VPN A-2
Customer MPLS
VPN network
Customer PE
CE 1
CE 2
VPN A
