Chapter 7. Troubleshooting Any Transport over MPLS Based VPNs

Any Transport over MPLS (AToM) is based on Internet Drafts draft-martini-l2circuit-trans-mpls and draft-martini-l2circuit-encap-mpls. AToM is a mechanism for transporting Layer 2 protocol data units (PDUs) over a multiprotocol label switching (MPLS) network. Supported Layer 2 PDU types include Ethernet, High-Level Data Link Control (HDLC), Point-to-Point Protocol (PPP), Frame Relay, and Asynchronous Transfer Mode (ATM) (AAL5 and cell relay). AToM allows the provisioning point-to-point circuits, which can then be used to construct Layer 2 VPNs.

Figure 7-1 illustrates AToM.

Figure 7-1. AToM

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In Figure 7-1, Frame Relay traffic is transported over the MPLS backbone between customer edge (CE) devices mjlnet_CE1 and mjlnet_CE3 via an emulated virtual circuit (pseudowire) with a virtual circuit (VC) ID of 126. Similarly, Frame Relay traffic is transported between mjlnet_CE1 and mjlnet_CE2 over a VC with VC ID 123.

Note that the label switched paths (LSPs) required for transport over the MPLS backbone are unidirectional in nature and, therefore, two LSPs are required between each pair of provider edge (PE) routers.