Introduction to Carrier Supporting Carrier (CSC)

I’m going to discuss with you today, the carrier supporting carrier “CSC” as in the previous post, we talked about how to connect VPN in two different locations from two different service providers with a clarification to the different inter-AS options.
Carrier Supporting Carrier solution enables small service providers to use the cloud of another large service provider in order to connect parts of their own networks; Actually, CSC takes inter-AS L3 VPN to the next level.

Carrier Supporting Carrier

CSC is a technology used to expand the reach of a SP by using another SP as transport, The service provider that provides connectivity over its cloud to the other providers is called the Backbone carrier and the service provider that uses the cloud is called the Customer carrier.

Backbone carrier which gives service to another carrier which is called customer carrier.

Customer carrier might be Provider or Enterprise Company which has its own VPNs.


The CSC-CE is the device located in the customer carriers network, connecting to the backbone carrier. The CSC-PE is the device located in the backbone carriers network connecting to the customer carrier.

The backbone carrier routers “CSC-PE” shouldn’t do any routing lookup, and thus should be substituted with label switching.

The backbone carrier carries the internal addresses of customer carrier. These addresses are loopback interfaces of PEs, RR loopbacks; But backbone carrier is not aware of the customer prefixes of customer carrier.

MPLS is always enabled for the CSC case but LDP is not must for assigning a label.

If IGP protocol runs between customer and backbone carrier then LDP is used. The rule is ”If prefix is learned from IGP, label should be assigned by LDP”.

If BGP protocol runs between customer and backbone carrier, like normal MPLS VPN PE-CE eBGP under the address-family ipv4 vrf on the CSC-PE side but adding the neighbor x.x.x.x send-label command and the mpls bgp forwarding interface command on both sides – When using eBGP for label exchange using the send-label option, mpls bgp forwarding is automatically configured under the interface.

Using BGP for CSC is the recommended solution, due to the following facts:

BGP takes the place of an IGP and LDP since we can use BGP to distribute routes with their MPLS labels, and certainly using a single protocol instead of two expedites the configuration and troubleshooting.

BGP is the preferred routing protocol for connecting two ISPs, mainly because of its wide scale routing policies and ability to scale.

By: Hatem Farag | CCIE#54446



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