Layer 2 circuits allow for the creation of point-to-point Layer 2 connections over an IP and Multiprotocol Label Switching (MPLS)-based network. Physical circuits with the same Layer 2 encapsulations can be connected together across such a network. Layer 2 circuits can allow for the replacement of end-to-end Asynchronous Transfer Mode (ATM) networks, Frame Relay networks, and some portions of Time-Division Multiplexing (TDM) networks, with an IP and MPLS-based network.
In Figure 71, an OC3/STM1 interface encapsulated with the Point-to-Point Protocol (PPP) on Router PE1 is connected over a Layer 2 circuit to reach an OC3/STM1 interface encapsulated with PPP on Router PE2. To enable the Layer 2 circuits to operate, the provider edge (PE) routers in Figure 71 are part of an MPLS network. Routers PE1 and PE2 must also be Label Distribution Protocol (LDP) peers. Additionally, any interface on the PE routers that connects to a customer edge (CE) router must support circuit cross-connect (CCC) interface encapsulations.
Figure 71: Layer 2 Circuit Connection
Layer 2 circuits are very similar to Layer 2 VPNs. However, there are some significant differences:
Layer 2 circuits are configured between two peers. The peers must use the same interior gateway protocol (IGP), such as Open Shortest Path First (OSPF) or Intermediate System-to-Intermediate System (IS-IS). Also, the peers must have a symmetrical Layer 2 configuration and belong to the same routing domain or autonomous system.
The basic building block for a Layer 2 circuit is a virtual circuit (VC). A VC is a point-to-point Layer 2 connection that is transported over MPLS or any other tunneling technology in a service provider network. A VC is similar to a CCC connection except that multiple VCs are transported over a single MPLS label-switched path (LSP) tunnel between two provider edge (PE) routers. In contrast, CCC only supports a single Layer 2 connection over a single LSP.
In Figure 72, the basic inner workings of Layer 2 circuits are explained. Two customer edge (CE) router logical interfaces, one local and one remote, are running the same Layer 2 protocol. Packets are sent from the local CE router to the remote CE router over an egress label advertised by the remote PE router. The label is transported over an LDP LSP (or LDP tunneled through RSVP) to the remote PE router that is connected to the remote CE router. Return traffic from the remote CE router is sent over an ingress label advertised by the local PE router. Once again, the label rides over an LDP LSP (or LDP tunneled through RSVP) to the local PE router from the remote PE router.
Figure 72: Layer 2 Circuit Concept
The Layer 2 circuit framework requires LDP to be used as the signaling protocol for advertising ingress labels. In most cases, it is not necessary to transport the Layer 2 encapsulation across the network; rather, the Layer 2 header can be stripped at one PE router, and reproduced at the egress PE router. Such Layer 2 information is carried in a special Layer 2 circuit header called a control word.
In the Layer 2 circuit IETF drafts, the control word is optional for most Layer 2 protocols, except Frame Relay and ATM AAL5 where it is required. However, in JUNOS Release 5.6 and later, a control word for all forms of Layer 2 circuits is sent by default. If you are establishing a Layer 2 circuit between a router running JUNOS Release 5.5 or earlier and a router running JUNOS Release 5.6 or later, use of the control word is negotiated automatically.
The Layer 2 protocols that are supported for Layer 2 circuits are:
For an Ethernet 802.1q VLAN or simple Ethernet, the entire Ethernet frame without the preamble or frame check sequence (FCS) is transported. For ATM cell-relay mode, ATM cells are transported without a SAR process. For Cisco HDLC, the frame is transported in its entirety except for HDLC flags and the FCS. For PPP, the frame is transported in its entirety except for any media-specific framing information.
For most protocols, a null control word consisting of all zeroes is sent between Layer 2 circuit neighbors. However, individual bits are available in a control word that can carry Layer 2 protocol control information. The control information is mapped into the control word, which allows the header of a Layer 2 protocol to be stripped from the frame. The remaining data and control word can be sent over the Layer 2 circuit, and the frame can be reassembled with the proper control information at the egress point of the circuit.
The Layer 2 protocols that map Layer 2 control information into special bit fields in the control word are as follows:
The JUNOS software implementation of sequence number processing for ATM cell-relay mode and AAL5 mode is not the same as that described in Sec. 3.1.2 of the IETF draft Encapsulation Methods for Transport of Layer 2 Frames Over MPLS Networks. The differences are as follows:
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