H3C Technologies H3C S7500E Series Switches - Manual (Page 238)

6-13
For good scalability, the Level 1 carrier does not inject the external routes of a Level 2 carrier; it only
injects routes for switching packets from different sites of the Level 2 carrier. The external routes
maintained by a Level 2 carrier are exchanged through BGP sessions established between related
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:
z
If the PE and the CE are in a same AS, you need to configure IGP and LDP between them.
z
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 6-11 , PE 3 and PE 4 exchange VPN routes of the Level 2 carrier through
IBGP sessions.
Figure 6-11 Scenario where the Level 2 carrier is an ISP
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
Figure 6-12 , PE 3 and PE 4 exchange VPN routes of the Level 2
carrier through MP-IBGP sessions.