Technical Tip: FortiSwitch disable-discovery not configurable on FortiOS v7.6.4 due to serial number length issue (Known Issue)

Description

This article describes a known issue in FortiOS v7.6.4 and later, where it is not possible to disable FortiSwitch discovery based on serial numbers due to an incorrect serial number check. Note that FortiOS v7.4 and earlier are not affected by this issue.

Scope

FortiOS v7.6.4 and later, Managed FortiSwitch.

Solution

The FortiGate can be configured to ignore FortiLink discovery of specific FortiSwitches visible on the network by configuring their serial numbers in the disable-discovery setting under config switch-controller global (see also: Converting to FortiSwitch standalone mode).

However, administrators will observe the following error message on FortiOS v7.6.4 and later when attempting to configure the disable-discovery setting:

FortiGate # config switch-controller global

FortiGate (global) # set disable-discovery S124FPTFXXXXXXXX
FortiSwitch(S124FPTFXXXXXXXX) serial number must be 36 characters long.
Command fail. Return code -651

This is a regression related to an earlier change (#1138430) made in FortiOS v7.6.4, where the length of the Managed FortiSwitch ID was increased from 16 characters to 35 to allow for more descriptive names (see also: New features or enhancements). The disable-discovery setting also utilized this length value, which resulted in the setting requiring each entry to be exactly 36 characters long (which is not possible for FortiSwitch serial numbers).

Change #1238963 has since been made to resolve this issue as of FortiOS v8.0.0 and v7.6.7. The fix will restore the character length limit back to 16 characters to match existing FortiSwitch serial numbers and allow disable-discovery to be configured correctly:

FortiGate # config switch-controller global 

FortiGate (global) # set disable-discovery aaaa
FortiSwitch(aaaa) serial number must be 16 characters long.
Command fail. Return code -651

FortiGate (global) # set disable-discovery S124FPTFXXXXXXXX

FortiGate (global) # show | grep -f disable-discovery
config switch-controller global
    set disable-discovery "S124FPTFXXXXXXXX" <---
end