A customer of an SP providing the BGP/MPLS IP VPN service may also be an SP. In this case, the SP providing the BGP/MPLS IP VPN service is called the provider carrier or the first carrier; the customer is called the customer carrier or the second carrier, as shown in Figure 1. This networking model is called carrier’s carrier. In this model, the second carrier SP is a VPN user of the first carrier SP.

Figure 1 Networking of carrier’s carrier

To ensure good extension, the second carrier adopts the operation mode similar to a stub VPN. That is, a CE of the first carrier advertises only the internal routes of the second carrier to the PE of the first carrier instead of advertising the routes of the CE’s customers. In this section, the routes of the VPN to which the CE belongs are called internal routes; the routes of the CE’s customers are called external routes.

The differences between the internal routes and external routes are as follows:

• The internal routes refer to the routes of the sites of the second carrier. The external routes refer to the routes of the customer sites of the second carrier, namely, the VPN routes of the second carrier.
• The internal routes are exchanged through BGP between the PEs of the first carrier. The external routes are exchanged through BGP between the PEs of the second carrier. The external routes are not advertised to the PEs of the first carrier.
• The VPN-IPv4 routes of the second carrier are regarded as external routes. The first carrier imports only the internal routes rather than the external routes of the second carrier to its VPN routing and forwarding table. This greatly diminishes the routes maintained on the first carrier network. The second carrier need maintain the internal routes and external routes.

Advantages and Disadvantages

The carrier’s carrier model has the following advantages:

• The second carrier is released from the burden of configuration, management, and maintenance, which are undertaken by the first carrier.
• The second carrier can flexibly plan addresses because its addresses are independent of the customers and the first carrier.
• The fist carrier can provide VPN services for multiple second carriers over a backbone network, and can provide Internet services at the same time. The profits of the first carrier, thus, are increased.
• The first carrier manages and maintains VPN services of each second carrier in the same manner instead of maintaining individual backbone networks for carriers. This simplifies the operation of the first carrier.

The carrier’s carrier model has the following disadvantages: As a strict symmetrical networking mode, only VPN users on the same level can communicate; users on different levels cannot communicate.

The VPN users on the same level need to directly exchange VPN routing information between each other. Therefore, the user devices on the same level must be routable. The users on this level must maintain the routing information of this level. The PEs on the same level need to directly exchange VPNv4 routing information between each other.

Implementation Principle

Compared with a common BGP/MPLS IP VPN, the access of first carrier CEs to first carrier PEs is the key to the carrier’s carrier model. A second carrier can be a common SP or an SP of the BGP/MPLS IP VPN service. First carrier CEs must run MPLS regardless of the role of a second carrier.

The implementation varies with the role of a second carrier.

• If a second carrier is a common SP, MPLS need not be configured on the PEs, which run IGP with the first carrier CEs. The second carrier PEs exchange external routes with each other through BGP sessions, as shown in Figure 2.

Figure 2 Common SP serving as the second carrier

• If a second carrier is a BGP/MPLS IP VPN SP, the PEs must support the MPLS functions. The PEs and the first carrier CEs run IGP and LDP. The second carrier PEs exchange external routes between each other through BGP sessions, as shown in Figure 3.

Figure 3 BGP/MPLS IP VPN SP serving as the second carrier

The following describes the routing information exchange and packet forwarding in the two situations.

Common SP Serving as a Second Carrier

If the second carrier is a common SP, there may be two cases:

• The first carrier’s backbone network and the second carrier’s network are located in the same AS.
  In this case, the first carrier PEs exchange routing information with the first carrier CEs through IGP and LDP. The first carrier CEs exchange external routing information between each other through BGP.
• The first carrier’s backbone network and the second carrier’s network are located in different ASs.
  In this case, the first carrier PEs exchange information about the labeled IPv4 routes with the first carrier CEs through BGP. The first carrier CEs exchange external routing information between each other through BGP.

In the two cases, packets are forwarded in the same manner.

BGP/MPLS IP VPN SP Serving as a Second Carrier

If a second carrier is a BGP/MPLS IP VPN SP, regardless of whether the first carrier’s backbone network and the second carrier’s network are located in the same AS or not, the following processes are required:

• LSPs of the public network must be set up between the PEs in the first carrier’s backbone network.
• LSPs of the public network must be set up between the PEs in the second carrier’s backbone network.

The differences are as follows:

• The protocols running between the first carrier PEs and the first carrier CEs are different.
• If the first carrier’s backbone network and the second carrier’s network are located in the same AS, the first carrier PEs and the first carrier CEs exchange routes through IGP and labels through LDP.
• If the first carrier’s backbone network and the second carrier’s network are located in different ASs, the first carrier PEs and the first carrier CEs exchange labeled routes through MP-EBGP.
• The number of labels carried in a VPN packet reaching the second carrier’s network.for the first time varies with networking scenarios:
• If the first carrier’s backbone network and the second carrier’s network are located in the same AS, a VPN packet carries two labels.
• If the first carrier’s backbone network and the second carrier’s network are located in different ASs, a VPN packet carries three labels.

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