RV130 blocking UDP 47808 traffic over pptp VPN

[Dougm]

Hello
I have a wan modem connected to a Cisco RV130 and enabled pptp VPN.
I can establish a secure connection though the WAN modem into the device via pptp.  I can ping devices on the LAN side of the RV130.  I can open local web pages for the configuration of any device on the LAN side of the RV130.  On the LAN network is a BACnet router that sends traffic over UDP 47808.  I cannot query that traffic.
Removed the WAN device and using my PC as if its the WAN I can still connect to the pptp server but I cannot see the BACnet traffic.
Using my PC connected to the LAN side I can observe all the traffic on the BACnet router on UDP port 47808.
I now know that my RV130 is blocking the traffic on UDP port 47808.  Why?  I have opened up a pptp VPN with a GRE tunnel.  All traffic should go through the tunnel.
Worried that maybe the GRE tunnel encapsulation may take another 24bytes I have configured all devices to be less than 1476.
Can anybody provide any assistance?

[Georg Pauwen]

Hello,

you might want to try and enable VPN Passthrough (page 108 of the attached admin guide)...

Also, make sure your RV130 is running the latest firmware version, release 1.0.3.45.

https://www.cisco.com/c/dam/en/us/td/docs/routers/csbr/rv130w/admin_guide/en/rv130w_admin_en.pdf

[Dougm]

The RV130 is acting as the VPN server and it cannot be used in the VPN pass through mode.  Do I still set it to VPN Pass though in this setup?

Doug

[joseph.h.nguyen]

Can you attach a network diagram?  Have you checked your firewall policy?  Typically PPTP is a layer 2 and GRE is a layer 3.  Your BACnet router may be forwarding its UDP traffic differently than expected.  

You can verify your MTU by using Windows ping command, "ping <ip address> -f -l <mtu size>".  Repeat this command with varying MTU size until you collect responsive and non-responsive pings.