4.1.g Describe basic layer 2 VPN —wireline

4.1.g [ii] ATOM general principals

Any Transport over MPLS (AToM) transports Layer 2 packets over a Multiprotocol Label Switching (MPLS) backbone. AToM uses a directed Label Distribution Protocol (LDP) session between edge routers for setting up and maintaining connections. Forwarding occurs through the use of two levels of labels, switching between the edge routers. The external label (tunnel label) routes the packet over the MPLS backbone to the egress Provider Edge (PE) at the ingress PE. The VC label is a de-multiplexing label that determines the connection at the tunnel endpoint (the particular egress interface on the egress PE as well as the virtual path identifier [VPI]/ virtual channel identifier [VCI] value for an ATM Adaptation Layer 5 [AAL5] protocol data unit [PDU], the data-link connection identifier [DLCI] value for a Frame Relay PDU, or the virtual LAN [VLAN] identifier for an Ethernet frame). EoMPLS carries frames inside MPLS packets.

Because the control word is included by default, so it may be necessary to explicitly disable this command in static pseudowire configurations. You can use mpls control-word command is used in static pseudowire configurations, the command must be configured the same way on both ends of the connection to work correctly. Otherwise, the provider edge routers cannot exchange control messages to negotiate inclusion or exclusion of the control word.

To specify the path that traffic uses a Multiprotocol Label Switching (MPLS) Traffic engineering (TE) tunnel or destination IP address and Domain Name Server (DNS) name), use the preferred-path command in pseudowire configuration mode. To disable tunnel selection, use the no form of this command.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4729-4736).  . Kindle Edition.