Before adding APN filtering to a GTP profile you must create an APN. You can also combine multiple APNs into an APN group. Use the following command to create an APN group:
config gtp apn
set apn <apn-value>
<apn-value> can include wild cards to match multiple APNs. For example
*.mcc333.mcn111.gprs would match all APNs from country
333 and carrier
111 on the gprs network.
From the GUI, go to Security Profiles > GTP APN and select Create New > GTP APN. Add a Name for the APN and add an APN Value.