Layer 2 Services over MPLS Implementation

When layer 2 services are configured over MPLS, layer 2 traffic is encapsulated in MPLS frames and sent over MPLS tunnels. A virtual circuit (VC) label that indicates a specific layer 2 connection, such as a Frame Relay data-link connection identifier (DLCI), is pushed into the label stack between the tunnel label and the layer 2 data.

A service-specific control word may be placed between the layer 2 data and the VC label. The control word is used for frame sequencing and carrying service-specific information, such as Frame Relay forward explicit congestion notification (FECN) and backward explicit congestion notification (BECN) information. At the tunnel end, the VC label is used to find the layer 2 interface over which the traffic is sent. The control word, if present, is used to convert the encapsulated layer 2 traffic into its native format.

Because MPLS labels are unidirectional, two VC labels are required for each layer 2 connection. The VC labels are distributed by the Label Distribution Protocol (LDP) in downstream-unsolicited (DU) mode between the two routers. The layer 2 connection status signaling may be emulated by advertising and withdrawing the VC labels. For example, if the Frame Relay subinterface between customer premises equipment (CPE) and a provider edge (PE) router goes down, the corresponding VC label is withdrawn by the PE router. When the remote PE router at the other end receives the label withdrawal, it translates the label withdrawal into LMI notifications to its CPE. When the Frame Relay subinterface comes back, a VC label is advertised, and the remote PE router again translates it into LMI notifications.

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