The DTP-1-ILGLCFG: Illegal config (on, isl--on,dot1q) on Port [mod/port] message can occur if both sides of the trunk are set to on, but the encapsulation types (isl, dot1q) do not match.
To resolve this issue, check the output of the show trunk command on both ends to ensure that the encapsulation types are the same.
If one end is set to on and isl and the other end is set to on and dot1q, the error message is generated because of the mismatch. Setting both ends to the same encapsulation should eliminate the error message.
Issue the set trunk command to configure the trunk and encapsulation.This is the syntax of the command:
set trunk[on|off|desirable|auto|nonegotiate] [vlan_range] [isl|dot1q|negotiate]
For example, to set a trunk on port 2/24 to on with 802.1q encapsulation, this would be the command:
set trunk 2/24 ondot1q
If both ends are set to on and have the same encapsulation, these messages should not be generated. If the messages continue, a communication failure may be occurring. Resetting the switch or reseating the line cards may be required to eliminate the error.