Hey Ibrahim,
Yes, both of these technologies are to support customer MPLS VPN sites.
Inter-AS is usually used when you need to connect to two different ISP's and have them provide you MPLS transport seamlessly. This would be the case if you have a site in the east coast of the US which connects to ISP1. This ISP1 is providing you MPLS transport, however they only provide MPLS transport to the east coast region. If you have another site in the west coast that connects to ISP2 which has the same issue (only provides MPLS transport to it's local region) you need a way to have MPLS transport across both ISP's to have end to end connectivity between your east coast and west coast sites.
Inter-AS allows this connectivity. Now ISP1 and ISP2 will communicate with each other to allow/share your MPLS information between the two. There are several ways this can be accomplished - InterAS option A,B,C and AB.
CsC is a bit different. We can use the above example - where you have an east coast site and a west coast site that need MPLS connectivity. However, in this example, your east coast and west coast site both connect to ISP1. ISP1 is slightly bigger and has presence in the east and west coast but does not have any presence in the middle states. In order to provide you MPLS transport end to end ISP1 will go through ISP2 which has presence in the middle states. Hence, we will only use ISP2 to get us connectivity to ISP1 in the opposite region.
Let me know if that answers your questions.
Victor Hugo Acevedo
