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Layer 2 Circuits Overview

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 1, 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 1 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 1: Layer 2 Circuit ConnectionLayer 2 Circuit Connection

Layer 2 circuits are very similar to Layer 2 VPNs. However, there are some significant differences:

  • You configure Layer 2 VPNs in a routing instance. As a result, Layer 2 VPNs have unique site and VPN identifiers. However, Layer 2 circuits do not require a routing instance configuration and instead use an alternate method of identifying circuits. Layer 2 circuit peer relationships are established with three components: a logical interface on the local PE router, the IP address of the remote PE router neighbor, and a virtual circuit identifier.

  • Layer 2 VPNs, like Layer 3 VPNs, require Border Gateway Protocol (BGP) for transport of traffic between PE routers. In contrast, Layer 2 circuits do not require BGP. Instead, Layer 2 circuits rely on LDP and MPLS for their operation. As a result, Layer 2 circuits require less configuration than Layer 2 VPNs.

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 2, 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 2: Layer 2 Circuit ConceptLayer 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 OS 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 OS Release 5.5 or earlier and a router running Junos OS 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:

  • ATM cell-relay mode and ATM Adaptation Layer 5 (AAL5) mode on ATM2 intelligent queuing (IQ) interfaces

  • Cisco High-Level Data Link Control (HDLC), Frame Relay, and PPP on SONET/SDH-based interfaces

  • Ethernet, VLAN, and Extended VLAN on Ethernet-based interfaces

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:

  • Frame Relay—This control word supports the transport of discard eligible (DE), forward explicit congestion notification (FECN), and backward explicit congestion notification (BECN) information.

  • ATM AAL5 mode—This control word supports the transport of sequence number processing, ATM cell loss priority (CLP), and explicit forward congestion indication (EFCI) information. When you configure an AAL5 mode Layer 2 circuit, the control information is carried by default and no additional configuration is needed.

  • ATM cell-relay mode—This control word supports sequence number processing only. When you configure a cell-relay mode Layer 2 circuit, the sequence number information is carried by default and no additional configuration is needed.

The Junos OS 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:

  • A packet with a sequence number of 0 is treated as out of sequence.

  • Any packet which does not have the next incremental sequence number is considered out of sequence.

  • When out-of-sequence packets arrive, the sequence number in the Layer 2 circuit control word increments by one and becomes the expected sequence number for the neighbor.