3Com S7906E Configuration Manual page 1528

routes of the Level 2 carrier. This can greatly reduce the number of routes maintained by the Level 1
carrier network.
Implementation of carrier's carrier
Compared with the common MPLS L3VPN, the carrier's carrier is different because of the way in which
a CE of a Level 1 carrier, that is, a Level 2 carrier, accesses a PE of the Level 1 carrier:
If the PE and the CE are in a same AS, you need to configure IGP and LDP between them.
If the PE and the CE are not in the same AS, you need to configure MP-EBGP to label the routes
exchanged between them.
In either case, you need to enable MPLS on the CE of the Level 1 carrier. Moreover, the CE holds the
VPN routes of the Level 2 carrier, but it does not advertise the routes to the PE of the Level 1 carrier; it
only exchanges the routes with other PEs of the Level 2 carrier.
A Level 2 carrier can be an ordinary ISP or an MPLS L3VPN service provider.
When the Level 2 carrier is an ordinary ISP, its PEs run IGP to communicate with the CEs, rather than
MPLS. As shown in Figure
IBGP sessions.
Figure 1-11 Scenario where the Level 2 carrier is an ISP
CE 1
Level 2 carrier
PE 3
When the Level 2 carrier is an MPLS L3VPN service provider, its PEs need to run IGP and LDP to
communicate with CEs. As shown in
carrier through MP-IBGP sessions.
1-11, PE 3 and PE 4 exchange VPN routes of the Level 2 carrier through
PE 1
MP-IBGP
Level 1 carrier
IGP/LDP/Labeled BGP
IGP IGP
IBGP
Figure 1-12, PE 3 and PE 4 exchange VPN routes of the Level 2
1-13
PE 2
CE 2
Level 2 carrier
PE 4