Benefits and Drawbacks of AToM-Based L2VPNs
Knowing that AToM pseudowires do a similar job to L2TPv3 pseudowires, you will not be surprised to learn that AToM-based L2VPNs share many advantages and disadvantages with L2TPv3-based L2VPNs.
Some of the main advantages for service providers implementing AToM-based L2VPNs are as follows:
AToM-based L2VPNs are MPLS based, and are therefore deployed over an MPLS backbone network. It is possible to implement AToM over a Generic Routing Encapsulation (GRE) tunnel (and an IP backbone), but this is not a typical method of implementation.
AToM allows the consolidation of service provider legacy and MPLS backbone networks. This consolidation of legacy and MPLS networks allows the service provider to save on operational and capital expenditure, while maintaining revenues for both legacy and IP/MPLS services.
Service providers can use AToM to deploy both legacy Layer 2 (Frame Relay and so on) and newer Ethernet service offerings.
Because AToM is MPLS based, a service provider can provision high service availability and traffic protection for AToM-based L2VPNs using MPLS traffic engineering (MPLS-TE) and MPLS-TE fast-reroute.
Some of the main advantages of AToM-based L2VPNs for enterprises are as follows:
AToM-based L2VPNs can be used to transport non-IP protocols such as Internetwork Packet Exchange (IPX) between enterprise sites.
Enterprises can maintain complete control of their routing when using AToM-based L2VPNsProvider Edge (PE) routers do not participate in enterprise routing (unlike with MPLS Layer 3 VPNs).
MPLS-based pseudowires can be used to provision both Virtual Private Wire Service (VPWS) and Virtual Private LAN Service (VPLS) architectures. Note, however, that although Label Distribution Protocol (LDP)-signaled MPLS pseudowires are used in both VPWS and VPLS architectures, the AToM command syntax described in this chapter is used for VPWS. For more information on VPLS, see Appendix A, "VPLS and IPLS Layer 2 VPNs."
Now that you understand the motivations behind the deployment of AToM-based L2VPNs, it is time to move on to a discussion of their underlying operation.