Control Word - HP HSR6800 Configuration Manual

If the peer PE requires the ingress to rewrite the P-tag:
The PE changes the P-Tag to the VLAN tag (the tag might be a null tag) expected by the peer PE, and then
encapsulates the packet. If the packet contains no P-tag, the PE adds a VLAN tag expected by the peer PE
(the tag value might be 0) and then encapsulates the packet.
For a packet sent to a CE, the PE rewrites, removes, or retains the P-tag depending on the
access mode you have configured.
NOTE:
A P-tag is also called a "service delimiter." An ISP may require users to add P-tags to distinguish
different user networks.
VC types for ATM links
For more information about ATM, see Layer 2—WAN Configuration Guide.
The VC type available for ATM links is ATM AAL5 transparent transport.
ATM AAL5 transparent transport is used to transparently transport ATM AAL5 Service Data Units
(SDUs) over a Packet Switched Network (PSN), such as an IP network, through an end-to-end
virtual connection. With this technology, two separate ATM networks can communicate with each
other through a PSN.
As shown in
the ATM cell, encapsulates the ATM AAL5 SDU, and then sends the ATM AAL5 SDU to PE 2 through
the VC on the PSN. PE 2 removes the outer encapsulation, encapsulates the original ATM AAL5
SDU to an ATM cell, and then sends the ATM cell to CE 2. In the communication process, CE 1 and
CE 2 cannot sense the existence of the PSN, as if they were communicating directly.
Figure 43 ATM AAL5 transparent transport

Control word

The control word field is between the VC label and the Layer 2 data. It carries information about the
Layer 2 packet, such as the sequence number.
The control word filed has the following functions:
•
Avoid packet disorder—In case of multi-path forwarding, packets received might be
disordered. You can configure the control word function on the device, so the device can
reorder the packets according to the sequence number carried in the control word field.
•
Transfer specific Layer 2 frame flags—When a PE processes Layer 2 packets, it might
discard some information, such as the FECN bit and BECN bit of Frame Relay. A PE can copy
such information to the control word field before sending Layer 2 packets to the peer PE, so the
peer PE can correctly process the Layer 2 packets.
•
Indicate payload length—If the payload of a packet to be transmitted on a VC is less than 64
bytes, a PE pads the packet to avoid transmission failure. The peer PE identifies the length of
Figure 43, after receiving an ATM cell from CE 1, PE 1 takes the ATM AAL5 SDU out of
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