I am trying, with my nexus 6P, to advertise an ibeacon. The format it seems correct but the signal is recognised by Android app and not from iPhone app. Do you know if there are some limitation?
The Nexus 6P supports BLE advertising, and you can make it transmit any known format for which you have the proper formula. You might want to try an off-the-shelf beacon transmitter app like Locate, which allows you to choose several formats, and see if your receiving device can pick up its transmission.
If the iOS device does not pick up the transmission, make sure the iOS device has been configured to range for the specific ProximityUUID that beacon is transmitting. Remember that iOS will not pick up all iBeacon transmissions -- it has to be pre-configured with the ProximityUUID first.
If you can make the iOS device pick up the Locate beacon transmission, but you can't make it pick up the transmission from your program, there may be an issue with the beacon format it is transmitting. Posting the code will help track that down.
Also, make very sure you set the BLE manufacturer code properly in the advertisement. iOS is very picky about the manufacturer code used when deciding whether to recognize a beacon advertisement. The Bluetooth SIG company code for Google may be appropriate for Eddystone beacons, but it should not be used for other beacon types they did not sponsor.