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For packets between customer networks to travel through the Level 1 carrier, the PE of the Level 1
carrier and the CE of the Level 2 carrier must assign labels to the backbone networks of the Level 2
carrier. The CE of the Level 2 carrier is a PE within the Level 2 carrier network.
Follow these guidelines to assign labels:
• If the PE and the CE are in the same AS, you must configure IGP and LDP between them. If
they are in different ASs, you must configure MP-EBGP to assign labels to IPv4 unicast routes
exchanged between them.
• You must enable MPLS on the CE of the Level 2 carrier regardless of whether the PE and CE
are in the same AS.
A Level 2 carrier can be an ordinary ISP or an MPLS L3VPN service provider.
As shown in
769HFigure 54, when the customer carrier is an ordinary ISP, its PEs and CEs run IGP to
communicate with each other. The PEs do not need to run MPLS. PE 3 and PE 4 exchange
customer network routes (IPv4 unicast routes) through an IBGP session.
Figure 54 Scenario where the Level 2 carrier is an ISP
As shown in 770HFigure 55, when the customer carrier is an MPLS L3VPN service provider, its PEs and
CEs must run IGP and LDP to communicate with each other. PE 3 and PE 4 exchange customer
network routes (VPN-IPv4 routes) through an MP-IBGP session.
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