the activation of the Gs interface also means that there are combined LA/RA updates.
“Co-ordination of LA update and RA update, including periodic updates, thus saving radio resources. A combined RA / LA update is sent from the MS to the SGSN. The SGSN forwards the LA update to the VLR.”
“When the MS enters a new RA in network operation mode I, the MS sends a Routeing Area Update Request message to the SGSN, as described in clause “Combined RA / LA Update Procedure”. The LA update is included in the RA update.”
“A routeing area update takes place when a GPRS-attached MS detects that it has entered a new RA, when the periodic RA update timer has expired, or when the MS has to indicate new access capabilities to the network”
check your RA Code and RA Colour design (I recommend 1 RA code and 1 RA colour per LAC)
(example of such settings : all cells in your network are set with RA code = 1 and RA colour = 1. It is strange, but actually each routing area is different from LAC to LAC, because the RA id is defined by the association LAC + RAC, which is here unique.)