AToM traffic encapsulation(Control Word)

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:

• 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. (For configuration information, see Option: Map Layer 2 Protocol Control Information into a Layer 2 Circuit.)
• 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.