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Understanding MPLS Layer 2 Circuits


An MPLS Layer 2 circuit is a point-to-point Layer 2 connection that transports traffic by means of MPLS or another tunneling technology on the service provider network. The Layer 2 circuit creates a virtual connection to direct traffic between two customer edge (CE) routers across a service provider network. The main difference between a Layer 2 VPN and a Layer 2 circuit is the method of setting up the virtual connection. As with a leased line, a Layer 2 circuit forwards all packets received from the local interface to the remote interface.

Each Layer 2 circuit is represented by the logical interface connecting the local provider edge (PE) router to the local CE router. All Layer 2 circuits using a particular remote PE router neighbor is identified by its IP address and is usually the endpoint destination for the label-switched path (LSP) tunnel transporting the Layer 2 circuit.

Each virtual circuit ID uniquely identifies the Layer 2 circuit among all the Layer 2 circuits to a specific neighbor. The key to identifying a particular Layer 2 circuit on a PE router is the neighbor address and the virtual circuit ID. Based on the virtual circuit ID and the neighbor relationship, an LDP label is bound to an LDP circuit. LDP uses the binding for sending traffic on that Layer 2 circuit to the remote CE router.