AGCH Channel via virtual Um

Hello all! I'm sorry for a stupid question, but I can't find an answer by myself. We are using Osmocom solution in University to demonstrate to students how GSM works. We have and fully virtualized workplaces with osm-bts-virtual and osmocom-bb UE via virtual Um. We are making an IMSI Attach procedure and capturing traces with wireshark. Virtual Um messages are visible in wireshark traces with "gsmtap" filter. According standart description, after RACH request UE should receive channel assignment via AGCH channel. But AGCH is not visible in traces. Am I wrong in my expectations somewhere? I tried to filter out messages, tried to filter gsmtap.chan_type == 4 but still no luck. Thank you Vania