Control Word - HP MSR2000 Configuration Manual

VLAN mode—A Layer 3 Ethernet subinterface or VLAN interface is bound to a PW. Packets
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received from the VLAN are forwarded through the bound PW. The peer PE can modify the VLAN
tag as needed. The default data encapsulation type for VLAN mode is VLAN.
PPP/HDLC over MPLS
PPP/HDLC over MPLS uses an MPLS L2VPN to connect a PPP or an HDLC network, and delivers PPP or
HDLC packets over the MPLS backbone through a PW.
If the AC type is PPP, the PW type is PPP. If the AC type is HDLC, the PW type is HDLC.
PPP/HDLC over MPLS only supports the port mode. You can use a command to associate a Layer 3
interface whose encapsulation type is PPP or HDLC with a PW.
In PPP encapsulation or HDLC encapsulation mode, a PE processes a PPP or HDLC packet as follows:
1. After receiving a packet from a Layer 3 interface, the PE searches for the PW bound to the
interface.
2. The PE encapsulates the packet and sends the packet to the peer PE through the PW.
3. The peer PE removes the outer encapsulation to get the original PPP or HDLC packet, and then
forwards the packet to the user network.

Control word

The control word field is between the MPLS label stack and the Layer 2 data. It carries control information
for the Layer 2 packet, such as the sequence number.
The control word field has the following functions:
Avoids packet disorder. In case of multipath forwarding, packets received might be disordered. You
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can configure the control word function so the device reorders the packets according to the
sequence number carried in the control word field.
Transfers specific Layer 2 frame flags, such as the FECN bit and BECN bit of Frame Relay.
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• Identifies the original payload length for packets that include padding.
When the PW type is FR DLCI or ATM AAL5 SDU VCC, packets on the PW always carry the control word
field, and the control word function cannot be disabled.
When the PW type is Ethernet or VLAN, the control word field is optional. You can configure whether to
carry the control word field in packets sent on the PW. If you enable the control word function on both PEs,
packets transmitted on the PW carry the control word field. Otherwise, the packets do not carry the
control word field.
Figure 91 Packet encapsulation in port mode
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