dear both, office have a old win 2003 which have VPN function.
I tested, if using PC/mac/iphone, is ok.
But if connect by andorid, the server events log, will show about GRE protocol blocked... , so I cannot connect by any android phone.
we are using tp-link 1043D , and this VPN is using over 2 years. But most of the user is using computer and Iphone, which no any problem. (I assume not relate to ISP/SIM card, because if using PC / Iphone, it is ok for same ISP or same telcom.)
Today,I Just test with android phone, it have this error.
之前讀書，試過某D 行新Android Version (e.g. Android 4.0 or above @2013)的電話駁唔返Campus 的L2TP IPSec VPN，要等IT staff 做野Update VPN Server/Router 至用到...
Port 500 UDP / Port 4500 UDP / Port 1701 UDP and protocol 50 (ESP)
Port 1723 / and protocol 47 (GRE)
BTW 推介Softether VPN Server，支援SSL, L2TP IPsec，open source，最重要官方定期出update，幾新的Android, IOS, Windows Client都用到VPN...