The control word command provides the option to add a control word as part of the packet encapsulation for PW types for which the control word is optional. These are Ethernet pseudowire (Epipe), ATM N:1 cell mode pseudowires (Apipe vc-types atm-vcc and atm-vpc), and VT pseudowire (Apipe vc-type atm-cell). The control word may be needed because when ECMP is enabled on the network, packets of a specific pseudowire may be spread over multiple ECMP paths if the hashing router mistakes the PW packet payload for an IPv4 or IPv6 packet. This occurs when the first nibble following the service label corresponds to a value of 4 or 6.
The control word negotiation procedures described in Section 6.2 of RFC 4447 are not supported and, therefore, the service only comes up if the same C-bit value is signaled in both directions. If a spoke-SDP is configured to use the control word, but the node receives a label mapping message with a C-bit clear, the node releases the label with an ‟Illegal C-bit” status code per Section 6.1 of RFC 4447. As soon as the user enables control of the remote peer, the remote peer withdraws its original label and sends a label mapping with the C-bit set to 1 and the VLL service is up in both nodes.
When the control word is enabled, VCCV packets also include the VCCV control word. In that case, the VCCV CC type 1 (OAM CW) is signaled in the VCCV parameter in the FEC. If the control word is disabled on the spoke-SDP, the Router Alert label is used. In that case, VCCV CC type 2 is signaled. For a multi-segment pseudowire (MS-PW), the CC type 1 is the only type supported; therefore, the control word must be enabled on the spoke SDP to be able to use VCCV-ping and VCCV-trace.
The following example shows a spoke SDP control word configuration:
-Dut-B>config>service>epipe# info
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description "Default epipe description for service id 2100"
sap 1/2/7:4 create
description "Default sap description for service id 2100"
exit
spoke-sdp 1:2001 create
control-word
exit
no shutdown
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*A:ALA-Dut-B>config>service>epipe#
To disable the control word on spoke-sdp 1:2001:
*A:ALA-Dut-B>config>service>epipe# info
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description "Default epipe description for service id 2100"
sap 1/2/7:4 create
description "Default sap description for service id 2100"
exit
spoke-sdp 1:2001 create
exit
no shutdown
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*A:ALA-Dut-B>config>service>epipe#