When one segment of the pseudowire cross-connect at the S-PE is static while the other is signaled using T-LDP, the S-PE operates much like a T-PE from a signaling perspective and as an S-PE from a data plane perspective.
The S-PE signals a label mapping message as soon as the local configuration is complete. The control word C-bit field in the pseudowire FEC is set to the value configured on the static spoke-SDP.
When the label mapping for the egress direction is also received from the T-LDP peer, and the information in the FEC matches that of the local configuration, the static-to-dynamic cross-connect is effected.
It is possible for end nodes of a static pseudowire segment to be misconfigured. In that case, an S-PE or T-PE node may be receiving packets with the wrong encapsulation. Therefore, an invalid payload will be forwarded over the pseudowire or the SAP, respectively. Also, if the S-PE or T-PE node is expecting the control word in the packet encapsulation, and the received packet comes with no control word but the first nibble below the label stack is 0x0001, the packet may be mistaken for a VCCV OAM packet and may be forwarded to the CPM. In that case, the CPM will perform a check of the IP header fields, such as version, IP header length, and checksum. If any of this fails, the VCCV packet will be discarded.