Disable the Control Word for Layer 2 VPNs

The emulated VC encapsulation for Layer 2 VPNs 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.

JUNOS software does not support the control word for any 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). This null control word is configured by default. If JUNOS receives a packet with a control word attached, the control word is discarded before the packet is forwarded to its destination.

JUNOS 5.5 and earlier releases do not support the control word at all. If you have configured Layer 2 VPNs on a network where some routers are running the current JUNOS release and some routers are running JUNOS 5.5 or earlier releases, you need to disable the control word on the routers running JUNOS 5.6 and later releases. To disable the control word, include the no-control-word statement at the [edit routing-instances routing-instance-name protocols l2vpn] hierarchy level on the routers running the current JUNOS release:

[edit routing-instances routing-instance-name protocols l2vpn]

no-control-word;

This is not necessary when configuring Layer 2 circuits. For more information, see Disable the Control Word for Layer 2 Circuits.