General Best Practices › Avoid Packet Duplication

Avoid Packet Duplication

The term packet duplication refers to reporting on the same traffic multiple times as it passes through interfaces on a switch. Several port mirroring configurations can result in duplication. The presence of duplicate packets can skew the metrics that are collected. Packet loss statistics are affected because duplicate packets are viewed as retransmissions.

As a best practice, configure mirror ports to minimize or eliminate duplicate packets. Another option for avoiding duplication is to mirror only packets traveling in the receive direction. This setup excludes traffic coming from clients into the VLAN.

When a VLAN sends all traffic to the mirror port, duplication occurs because traffic from all ports is forwarded to Application Delivery Analysis. In cases where packet duplication is likely, consider mirroring individual ports rather than whole VLANS. With this technique, only individual ports or interfaces are used as mirror sources. Only packets destined for selected servers are sent to the mirror port. Use the show command to see a list of all ports included in a VLAN.

Deduplication logic applies to all packets received on a given logical port. Therefore, if a duplicate packet from the same VLAN is received on a different logical port, it is not discarded. If you combine two physical ports into a single logical port definition, a duplicate is discarded in the following situations:

• If it arrives on a physical port within a few packets of the original packet on the other physical port
• If it arrives on a second switch.

Both packets are retained if the two physical ports are not combined into a logical port.

Note: The CA Application Delivery Analysis Administrator Guide provides instruction for deduplicating TCP packets on a single-port monitor or standalone manager.