# Multi-Link packet duplication

The multi-link packet duplication feature enables duplication of traffic packets over multiple links that are sent to the same destination. This eliminates all packet loss due to link failure or delay in packet loss detection

Requirements:

- There must be at least two available links in active mode to the destination gateway.
- The destination gateway must run the engine version that is supported by the packet duplication feature.
- The packet duplication feature is only supported on engine version 7.2 and later.

Important: It is recommended to use this feature only for critical application traffic that is highly sensitive to packet loss and when normal Multi-Link VPN setup packet loss detection is not fast enough to select alternative link on a link failure or on sudden increase in packet loss. Note that when the feature is enabled, it doubles the network bandwidth usage for the traffic selected for packet duplication.
The multi-link packet duplication feature can be enabled by:

1. Selecting what QoS classes are enabled for packet duplication. For more details, refer to the Create Link Usage Profile elements topic.
2. (Optional) Controlling what link types should not be used for packet duplication. For more details, refer to the Create Link Usage Profile elements topic.
3. Using the configured Link Usage Profile in the engine configuration. For more details, refer to the Select a Link Usage Profile element for an Engine topic.
4. Selecting the QoS class in the access rule to match the traffic for multi-link packet duplication. For more details, refer to the Use QoS Class elements to apply custom link selection options to traffic topic.

Note:

- The receiving gateway forwards only the first received packet. The duplicate packet received later is dropped.
- The link selection for packet duplication is done automatically.
- The maximum transmission unit (MTU) size for the connections subjected to packet duplication is slightly smaller than the MTU size with the normal VPN.
- The configuration done must match in both the tunnel endpoint engines to achieve symmetric packet duplication for both sent and received packets of the critical connections.