Fortigate GRE not standing up over 3G/4G PPPoE

[TigerstripeNZ]

Fortigate 310B -- Sierra Wireless MP70 3G/4G Modem -- Internet -- Fortigate 310B

I am currently experiencing an issue where I am able to ping both sides of the network (public facing IP addresses) but unable to stand up a GRE between the two sites.

[ul]

If I change my bearer from the MP70 to a terrestrial ISP link, the GRE stands up.

I have connectivity to the internet suing the MP70

I am able to ping both public interfaces

My GRE and VPN settings are correct and proven

I run OSPF for auto switching of bearers

Diag sniffer packet shows the following output...[/ul]

2.970050 x.x.18.253 -> x.x.2.234 ip-proto-47 68,...

[ul]

MTU on both end Fortigates set to 1500, MSS set to 1476.

MTU on MP70 set to 1438

MP70 set up as a pass-through device[/ul]

Could this be an issue with MTU? both sites are currently set @ MTU 1500. I understand that there are additions to the packet when using GRE/IPsec and PPPoE, and MTU adjustments may need to be made.

Is there any reason why GRE would not work over a 3G/4G PPPoE link, has anyone experienced this before?

I am still working through engineering this, but if anyone has any out of the box ideas that I wouldn't think of exploring; hearing from you would be fantastic!

[Lokeshmangla]

Hi All.

I am trying to connect PPTP from client PC to PPTP server but can not complete connection and error code is 806.

on further throubleshooting , I found PPTP packets are getting passed but issues seems in GRE packets. 

If I bypass 1500D then I am able to connect PPTP server.

[emnoc]

What does a MTU path ping discovery show, at the minimum you will need to reduce the MTU by 24+8 bytes for GRE and  PPPoE.

I would start with the Path MTU and then make adjustments

Ken Felix