What does Illegal Carrier do?

[Q] In CPS I have read a codeplug from our SLR 5700 (SLR5500) and under general settings it has an option for "Illegal carrier feature enable". I understand that in P25 systems at least when an Illegal carrier is detected the repeater will fallback to a back up frequency to avoid jamming, does this work the same in Mototrbo? because maybe I'm missing something but after I enable it, there is no where to input a fallback frequency.

[A] I know almost nothing about P25/Astro. There is nothing like that in DMR.

Illegal Carrier in the SLR repeaters is only there for system monitoring purposes. When enabled, if an interfering signal is detected for longer than the Illegal Carrier Timer with a signal strength of more than the Illegal Carrier Threshold, the repeater will  and generate an alarm. The severity of the alarm can be configured to be either Major; Minor or Informational.

The repeater regards a DMR transmission with an incorrect Colour Code or any other non-DMR signal as an illegal carrier. 

The repeater behavior depends what Illegal Carrier Alarm is set to: if set to Informational, the repeater may continue to operate normally; if set to Major, the repeater will be taken out of action until it gets reset.

When I say "may continue to operate" it means that the repeater will transmit signals but reception could be impacted, depending on how strong the interference is and for how long it is present.

If the repeater is part of a Capacity Plus or Capacity Max system and Illegal Carrier Alarm is set to anything other than Informational, it will remove itself from contention for being a Rest Channel or Control Channel host whenever there is interference. If an illegal carrier is detected by a control channel host, this function will be transferred to the next available control channel capable repeater.

Another setting that may be of interest is RSSI Threshold (in the personality set).