Layer 2 Circuit Overview
A Layer 2 circuit is a point-to-point Layer 2 connection transported by means of Multiprotocol Label Switching (MPLS) or another tunneling technology on the service provider's network. A Layer 2 circuit is similar to a circuit cross-connect (CCC) except that multiple Layer 2 circuits can be transported over a single label-switched path (LSP) tunnel between two provider edge (PE) routers. In contrast, each CCC requires a dedicated LSP.
The JUNOS software implementation of Layer 2 circuits supports only the remote form of a Layer 2 circuit; that is, a connection from a local customer edge (CE) router to a remote CE router. Figure 45 illustrates the components of a Layer 2 circuit.
The interfaces shown in Figure 45 are logical interfaces. Packets are sent to the remote CE router using an egress virtual private network (VPN) label advertised by the remote PE router. The VPN label transits over either a Resource Reservation Protocol (RSVP) or a Label Distribution Protocol (LDP) LSP (or other type) tunnel to the remote PE router connected to the remote CE router. If you configure RSVP for Layer 2 Circuits, you must also configure LDP.
Return traffic sent from the remote CE router to the local CE router uses an ingress VPN label advertised by the local PE router, which again transits over an RSVP and LDP LSP to the local PE router from the remote PE router. LDP is the signaling protocol used for advertising VPN labels.