Carrier supporting carrier (CSC) feature using the IP Solution Center (ISC) provisioning process

To configure the CSC network to exchange routes and carry labels between the backbone carrier provider edge (CSC-PE) routers and the customer carrier customer edge (CSC-CE) routers, use Label Distribution Protocol (LDP) to carry the labels and an Internal Gateway Protocol (IGP) to carry the routes.

• LDP/IGP

A routing protocol is required between the CSC-PE and CSC-CE routers that connect the backbone carrier to the customer carrier. The routing protocol enables the customer carrier to exchange IGP routing information with the backbone carrier. RIP, OSPF, or static routing as the routing protocol can be selected.

Label distribution protocol (LDP) is required between the CSC-PE and CSC-CE routers that connect the backbone carrier to the customer carrier. LDP is also required on the CSC-PE to CSC-CE interface for VPN routing/forwarding (VRF).

• IPv4 BGP Label Distribution

BGP takes the place of an IGP and LDP in a VPN forwarding/routing instance (VRF) table. You can use BGP to distribute routes and MPLS labels. Using a single protocol instead of two simplifies the configuration and troubleshooting.

BGP is the preferred routing protocol for connecting two ISPs, mainly because of its routing policies and ability to scale. ISPs commonly use BGP between two providers. This feature enables those ISPs to use BGP.

When BGP (both EBGP and IBGP) distributes a route, it can also distribute an MPLS label that is mapped to that route. The MPLS label mapping information for the route is carried in the BGP update message that contains the information about the route. If the next hop is not changed, the label is preserved.
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