3Com MSR 50 Series Configuration Manual page 1469

In either case, you must enable MPLS on the CE of the Level 2 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 a 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
routes of the Level 2 carrier through IBGP sessions.
Figure 411 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
exchange VPN routes of the Level 2 carrier through MP-IBGP sessions.
Figure 412 Scenario where the Level 2 carrier is an MPLS L3VPN service provider
CE 1
Level 2 carrier
PE 3
Figure 411, PE 3 and PE 4 exchange VPN
PE 1
MP-IBGP
Level 1 carrier
IGP/LDP/Labeled BGP
IGP IGP
IBGP
PE 1
MP-IBGP
Level 1 carrier
IGP/LDP/Labeled BGP
IGP/LDP IGP/LDP
MP-IBGP
MPLS L3VPN Overview
PE 2
CE 2
Level 2 carrier
Figure 412, PE 3 and PE 4
PE 2
CE 2
Level 2 carrier
1469
PE 4
PE 4