Control Word - HP HPE FlexNetwork MSR Router Series Configuration Manual

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 frame, for example, the sequence number.
The control word feature has the following functions:
•
Avoids packet disorder. In case of multipath forwarding, packets received might be disordered.
The control word feature 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.
•
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 feature 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 feature on
both PEs, packets transmitted on the PW carry the control word field. Otherwise, the packets do not
carry the control word field.
MPLS L2VPN interworking
CEs might connect to PEs through various types of links, such as ATM, FR, HDLC, Ethernet, and
PPP. MPLS L2VPN interworking connects such CEs and allow them to communicate.
MPLS L2VPN supports Ethernet interworking and IP interworking modes. The device only supports
IP interworking. Only local MPLS L2VPN connections, static PWs, LDP PWs, and remote CCC
connections support the interworking feature.
Figure 103 Ethernet to PPP interworking
Eth header IP header
As shown in
1. CE 1 sends an Ethernet frame destined for CE 2 to PE 1.
2. PE 1 checks whether the packet encapsulated in the received Ethernet frame is an IP packet.
If yes, PE 1 removes the Ethernet header, adds PW label V and tunnel label T to the IP
packet, and forwards the packet to PE 2 through the tunnel.
If not, PE 1 drops the frame.
3. PE 2 obtains the output interface according to the PW label V in the received packet, removes
the PW label, adds an PPP header, and forwards the PPP frame through the output interface to
CE 2.
T V
PE 1
IP data
Ethernet link
CE 1
Figure 103, a packet in an MPLS L2VPN interworking scenario is forwarded as follows:
MPLS or IP backbone
IP header IP data
409
V: PW label
T: Tunnel identifier
PE 2
PPP header IP header
PPP link
CE 2
IP data