Troubleshooting Tip: LACP does not distributed evenly on L4 algorithm after upgrade to v7.2.5 onwards or v7.4.0 onwards

Description

This article describes how to troubleshoot LACP uneven distribution of packets on the L4 algorithm after upgrading to v7.2.5 onwards or v7.4.0 onwards

Scope

FortiOS v7.2.x, FortiOS v7.4.x, FortiOS v7.6.x., FortiOS v8.0.

Solution

The following command has been used to check the distribution of the packet:

# diagnose hardware deviceinfo nic port4
# diagnose hardware deviceinfo nic port5
# fnsysctl ifconfig -a port4
# fnsysctl ifconfig -a port5

Before v7.2.5, the output showed below with the calculation of the difference on Rx Bytes and Tx Bytes.

1st interface:

Rx Bytes: 38026-21406=16620.
Tx Bytes: 89535761-4315=89531446.

2nd interface:

Rx Bytes: 138109-8496=129613.
Tx Bytes: 106875101-2615=106872486.

After the upgrade to v7.4.x:

1st interface:

Rx Bytes: 13754798-13740365=14424.
Tx Bytes: 787302386-591453888=195848498.

2nd interface:
Rx Bytes: 3328521-3199447=129074.
Tx Bytes: 3225837-2670681=555156.

The Tx Bytes have a particularly large difference before and after the update. After the upgrade to v 7.4.x, the ratio on the 1st interface is much larger

This was due to a change on bug ID 0838036

Workaround:

Using the L3 algorithm on the LACP interface.