So first of all: Chromebook is on version 47.0.2526.106 platform 7520.63.0 and Firmware Google_Snow.2695.117.0

2860n+ settings [VPN & Remote Access]
Remote Access Control: Tick enable IPSec VPN service & Enable L2TP VPN service
PPP General Setup: Don't think this is relevant but I do have 'Mutual Authentication (PAP) as 'Yes' along with username & password [probably a legacy from when I used to have PPTP running...?]
IPsec General Setup: Certificate for dial in - None
Preshared key - I have a 'random' phrase which is also required on the Chromebook
IPsec Security Method - Tick all [Medium AH, High DES/3DES/AES]
IPsec Peer Identity: nothing entered
Remote Dial-in User: Index 1 is active settings are:
- User account & authentication - enabled, idle timeout 300 [but you may not want this value]
- Allowed dial in type L2TP (only) with 'Must' have IPsec policy
- Specify remote node - not used [Netbios & Multicast set to 'Pass']
- Subnet left at default LAN 1
- Username & Password - set to VPN login credentials and replicated on Chromebook
- IKE Authentication Method - Left at default
- IPsec Security Method - ticked all AH/DES/3DES/AES
LAN to LAN - Not used. All blank
VPN trunk Management - Not used (default)
Connection Management - Dial out tool not used. VPN Connection status shows connected!!!

I think that's all... Of course you may have to reboot your router, I can never quite tell which changes require reboots and which don't.

Good luck.

Chris