Disable the Control Word for Layer 2 Circuits

The emulated VC encapsulation for Layer 2 circuits is accomplished by adding a 4-byte control word between the Layer 2 protocol data unit (PDU) being transported and the VC label that is used for demultiplexing. Various networking formats (ATM, Frame Relay, Ethernet, and so on) use the control word in a variety of ways.

The JUNOS software supports the control word for Frame Relay. However, it does not support the control word for any other networking format, meaning that it is not fully compliant with the Internet draft in cases where the control word is mandatory. To be minimally compliant with the Internet draft, JUNOS supports a null control word (a control word of all zeros). If JUNOS receives a packet with a control word attached, the control word is discarded before the packet is forwarded to its destination.

JUNOS can typically determine whether a neighboring router supports the control word. However, if you want to explicitly disable its use on a specific interface, include the no-control-word statement:

no-control-word;

You can configure this statement at the following hierarchy levels:

• [edit logical-routers logical-router-name protocols l2circuit neighbor address interface interface-name]
• [edit protocols l2circuit neighbor address interface interface-name]

This statement might be required for Layer 2 VPN configurations. For more information, see Disable the Control Word for Layer 2 VPNs.