Inter-Autonomous System Connectivity: Another Application of Tunnels

Carrier Supporting Carrier

Carrier supporting carrier (CsC) is a two-layer IP VPN solution designed to allow a backbone carrier to use MPLS VPN (or L2TPv3) to carry traffic belonging to customers' carriers that use MPLS VPNs.

Before looking at the solution, it is a good idea to understand the problem being solved . An MPLS PE router holds all the routes of all the sites to which it connects. In a normal scenario, although this number can be large, the expectation is that an individual VPN would require at most hundreds or perhaps thousands of entries in a VRF. However, if the customer is itself an ISP, carrying routes belonging to their customers, the potential exists to require the backbone PE to carry an impossibly large number of routes. The CsC solution addresses this issue.

CsC is based on the observation that the label switched domain of an MPLS VPN network (that is, the backbone network) only needs routing information to reach provider (P) routersthe customer routing domain is invisible to the core.

In a CsC scenario, the ISP needs to share the global routing table with the backbone carrier only. The CsC backbone routers (labeled CSC-PE1, CSC-P, and CSC-PE2 in Figure 5-13) carry the next-hop routes for the ISP carrier networks so that an LSP exists between ISP sites. Note that the next -hop routes should not be aggregated because that would break the end-to-end LSP.

Figure 5-13. CsC Topology

Figure 5-13 shows the CsC topology.

The major differences between CsC and a standard MPLS VPN solution are as follows :

• CE-PE interface use MPLS.
• CE-PE exchange routes and labels.
• Packets on the CsC backbone have three labels on their label stack.

Figure 5-13 shows CsC data-plane operation, specifically the label stack of a packet as it traverses the ISP and backbone carrier networks:

1. CE1 sends a packet with a destination address in the 10.2.0.0/24 network. The next hop for this address is PE1.
2. PE1 pushes two labels: the VPN identifier, 20, and the next-hop label announced by P1, 31.
3. P1 does a label swap and forwards the packet with outer label value of 33.
4. CSC-CE1 does a label swap and forwards the packet with outer label 26. This label was announced by CSC-PE1.
5. CSC-PE1 removes label 26 and pushes two labels onto the stack. Label 19 is the VPN identifier that identifies the ISP's VRF. Label 36 is the value announced by the CSC-P router, which is the next hop on the CSC backbone network.
6. CSC-P performs a PHP operation and forwards the packet with outer label value 19.
7. CSC-PE2 matches the incoming label value to the correct VRF and pushes label 48 before forwarding to CSC-CE2.
8. CSC-CE2 does a PHP and forwards the packet with outer label value of 20.
9. PE2 matches the incoming label value to the correct VRF and forwards an IP packet to the customer router, CE2.

Figure 5-13 also illustrates the control-plane operation:

1. PE1 and PE2 exchange labels and VPNv4 routes using MP-BGP. The labels identify customer VRFs.
2. PE1, P1, and CSC-CE1 exchange labels using LDP. These labels identify the next-hop FEC.
3. CSC-CE1 and CSC-PE1 exchange labels and routes. There are two ways to do this. The first uses LDP; the second, specified in RFC 3107, uses external BGP (eBGP) to exchange IPv4 and labels.
4. CSC-PE1 and CSC-PE2 exchange labels and VPNv4 routes using MP-BGP. These labels identify customer VRFs.
5. CSC-PE1, CSC-P, and CSC-PE2 exchange labels using LDP.