Wednesday, May 13, 2009

Control Word In Pseudowire

Control word plays a vital role in AToM. It is 32 bit field which is inserted between VC label and transport layer in case of AToM. This is added by the ingress PE and removed by the egress PE.

Structure of control Word

Tunnel Lable/IGP Label
VC label
Control Word
Layer 2 Frame

shivlu jain

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Jatil Thakore said...

Hi Shivlu,

Yes, you said is right, but I want to add more to your topic.

First, if the peer PE(either ingress or egress) does not support a control word, the control word is disabled and this negotiation is done by LDP label binding.

Second, the control word is optional for Ethernet, PPP, HDLC, and cell relay transport types. However, the control word is required for Frame Relay and ATM AAL5 transport types.

Jatil Thakore

Shivlu Jain said...

Hi Jatil

You are very much right.

If I am not mistaken you are ex-tulipian.

shivlu jain

bnyuan said...

Hi Shivlu,
You are all right.
I think the control word is optional for Frame-relay DLCI-to-DLCI mode and ATM VC-to-VC mode,because IP protocol has been useed IP-Length segment.


Usman said...


I heard that some vendors use Control-word to perform what is known as a "pseudowire traceroute". This helps in determining the exact path a PW will take through the provider network (especially when there is ECMP).
Can someone highlight this process in more detail as to how the control word helps in this PW traceroute ?