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How to configure the MPLS VPN Carrier supporting Carrier setup

Core Issue

A Multiprotocol Label Switching (MPLS) Virtual Private Network (VPN) enables service providers to create scalable and efficient Layer 3 (L3) VPNs. These combine the best features of overlay and peer-to-peer models. The Carrier supporting Carrier (CsC) feature builds over the basic MPLS VPN model and allows a single backbone carrier to interconnect different Point Of Presence (POP) sites of a customer carrier. A customer carrier is a service provider to which the end customers are directly connected.

The CsC feature provides these benefits:

  • A single CsC backbone is used to connect to many POP sites. The same backbone is used for interconnecting POP sites that belong to different customer carriers. The MPLS and VPN technique is used to separate traffic from different carrier's POP sites.
  • A single CsC backbone provides services to both VPN service providers and to Internet Service Providers (ISPs).
  • The customer carriers do not have to operate their own long distance network. They can purchase that service from the CsC backbone carrier.
  • Any link type supported by MPLS can be used inside the CsC backbone and on access links between the CsC backbone and customer carriers.


CsC architecture is designed so that the CsC backbone only needs to know about routes available within the customer carrier. CsC does not need to know about routes that belong to end customers connected to customer carriers. This makes CsC architecture highly scalable.

The CsC backbone provides MPLS and VPN service where the customer carriers are connected as VPN sites. The routers in the different sites of the customer carrier exchange the end customer routes. These routes are exchanged through internal Border Gateway Protocol (iBGP) sessions directly between them.

The difference between a normal MPLS and VPN service and the CsC service is that the CsC service uses MPLS on the access link between the CsC Provider Edge (PE) router and the customer carrier's VPN site. In addition, label information exchange is required between the CsC PE router and the customer carrier's Customer Edge (CE) routers. This is because the packets are forwarded through the CsC backbone, which is not aware of the end customer routes. Label Distribution Protocol (LDP) can be used for this purpose.

The PE routers in the CsC backbone use label stitching to associate the VPN label for a network in the customer carrier to another label. These routers distribute label stitching to the CE router of the customer carrier through LDP. When using normal MPLS and BGP interaction, the CE router of a customer carrier uses this label (learned through LDP) as the next-hop label for customer routes. The customer routes are learned through the iBGP session from another site. This process extends the Label Switched Path (LSP) of a destination from a CE router in one site of a customer carrier to a CE router in another site of the same customer carrier.

Depending on the customer carrier capabilities, there are three different ways in which a CsC network can be set up. All of them require the CsC backbone carrier to be set up for MPLS VPN, require the CsC backbone to learn only about the routes in the customer carrier and not the end customer routes, and use MPLS on the access link between the CsC PE router and the CE router of customer carrier.

These are the three different ways a CsC network can be set up:

  • The customer carrier does not run MPLS inside its POP sites. All the routers within the customer carrier are required to know about the end customer routes because only normal IP packets are forwarded within their network. Fully meshed iBGP sessions, or other methods like route reflectors, are necessary for this purpose.
  • The customer carrier runs MPLS inside its POP sites. MPLS is used to distribute labels and forward labeled packets between customer carrier routers in each POP site. All customer carrier routers do not need to run iBGP. Only the Autonomous System Boundary Routers (ASBRs) connected to the end customers at each POP site need to run iBGP between them. Other routers within the customer carrier use normal MPLS BGP interaction. This is done to use the label for the BGP next-hop address as the label for a destination in the end customer network. In this case, the LSP of a destination extends from the ASBR router in one site of a customer carrier to the ASBR router in another site of the same customer carrier.
  • The customer carrier provides MPLS and VPN services to end users. This is similar to the previous method, but the customer carriers themselves use MPLS VPN. This offers VPN services to end customers. The ASBR routers in the customer carrier that connect to the end customers act as PE routers for the customer carrier. These routers use Multiprotocol iBGP sessions with ASBR and PE routers in other POP sites. This exchanges end customer VPN routes and labels. The LSP of a destination extends from the ASBR router in one site of a customer carrier to the ASBR router in another site of the same customer carrier. This is called hierarchical VPNs because there are two levels of VPNs. One level is provided by the CsC backbone for the customer carriers. The other level is provided by the customer carriers for the end customers.

Configuring a CsC can be broken into these steps:

  1. Before enabling the CsC feature, the MPLS VPN configuration has to be set up in the CsC backbone.

    For more information on setting up an MPLS VPN network, refer to these documents:

  2. To enable the CsC feature, perform these steps:
    1. Create a loopback interface inside the VPN Routing and Forwarding (VRF) on the CsC PE router. This interface is required for LDP operation inside the VRF. LDP uses the IP address on this interface to create a label space identifier. It is also used as the LDP transport address. Therefore, it must be advertised to the CE router using a supported Interior Gateway Protocol (IGP) as the PE-CE routing protocol.
    2. Configure MPLS under the interface on the CsC PE router that connects to the CE router of the customer carrier by issuing the mpls ip command in interface configuration mode. This interface is already a part of the VRF for that specific customer carrier because of the MPLS VPN configuration on the CsC PE router.
    3. Use LDP as the label distribution protocol on the same interface by issuing the mpls label protocol ldp command.
    4. Configure the CsC PE router to exchange routes with the CE router of the customer carrier by using any of the supported IGPs. This can be any IGP supported for PE-CE routing in an MPLS VPN setup and can be the same as the one used within the attached customer carrier sites.
    5. Repeat the previous steps on the CE router of the customer carrier connecting to the CsC PE router. Because the CE router is not MPLS VPN aware, it does not require VRF and other MPLS VPN configurations required on the PE router.
  3. Depending on the method used within the customer carrier network, configure the IGP, iBGP and optionally basic MPLS (for optimizing BGP sessions) or MPLS VPN configuration (for VPN service to end-customers).

For more information on CsC feature with configuration examples, refer to MPLS VPN Carrier Supporting Carrier.

Problem Type

MultiProtocol Label Switching (MPLS) issues


RP (Routing Protocol) Related Technologies

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‎06-22-2009 05:30 PM
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