ETHERNET Control Word

[romccallum]

Ok I know that the Control word in the likes of frame-relay carries the fecn and becns bits and more and ATM uses the control word to carry things like the CLP bit but what I cant really find is PURE evidence that Cisco kit utilises the ethernet control word. Its an optional byte for ethernet which in my reading is really only used if you require strict sequencing of the ethernet packets.

Can anyone tell me more about the Ethernet control word from a purely Cisco point of view i.e. is it used?

[Harold Ritter · Accepted solution]

Rob,

As per draft-martini-l2circuit-encap-mpls-09.txt, the control word is optional for the following vc-types:

TYPE_ETHER_VLAN (0x0004)

TYPE_ETHER (0x0005)

TYPE_HDLC (0x0006)

TYPE_PPP (0x0007)

Here's the excerpt from the RFC:

"The control word is OPTIONAL. If the control word is used then the flag bits in the control word are not used, and MUST be set to 0 when transmitting, and MUST be ignored upon receipt."

As far as our implementation is concerned, we include a default CW (all zeroes) unless we receive a vc label binding without a CW from the remote peer.

Hope this helps,

[romccallum]

Cheers H - had a sneaky suspicion it might be you that responds to this one. Know what I hate about this word - frame/ATM. All docs quote this "The control word is REQUIRED - oh but its use is optional" So if its use is optional then you could argue that it is NOT required. Damn english ;-)

Cheers

[Harold Ritter]

The use in the control word is indeed required when it comes to FR and ATM.

Cheers,