R3303-HP HSR6800 Routers MPLS Configuration Guide

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• 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 the padding
string according to the payload length field of the control word, so as to abstract the correct original
payload of the packet.
For some VC types, such as FR DLCI and ATM AAL5 transparent transport, packets transmitted on a VC
always carry the control word field. For some VC types, such as PPP, HDLC, FR port mode, the control
word field is optional, and you can configure whether to carry the control word field in packets. If you
enable the control word function on both PEs of a VC, packets transmitted on the VC carry the control
word field. Otherwise, packets do not carry the control word field.
VC redundancy
Only SVC and Martini VCs support VC redundancy.
If only one VC exists between two CEs, when this VC fails, the two CEs cannot communicate. With VC
redundancy, as shown in Figure 44, t
wo VCs (one primary and one backup) are established between CE
1 and CE 2. The CEs communicate through the primary VC. When the primary VC goes down, they
continue to communicate through the backup VC. More specifically, when PE 1 detects the failure of the
primary VC, it brings up the backup VC and then forwards packets from CE 1 to CE 2 through the
backup VC. When CE 2 receives the packet, it updates its MAC address table, so that packets from CE
2 travel through the backup VC, too.
Figure 44 VC redundancy protection in an MPLS L2VPN
The MPLS L2VPN determines whether the primary VC fails according to the LDP session status and the
BFD result. The backup VC is used in the following circumstances:
• The tunnel of the primary VC is deleted or no longer satisfies the tunneling policy, causing the
primary VC to go down.
• The BFD protocol detects that the primary VC has failed.