No internet with Asus RT-AC68U router and Zyxel router in modem mode

[Psychophile]

Hi all

I am trying to use my Asus RT-AC68U router on the Vodafone VDSL2 service, by connecting it via a Zyxel VMG3925 B10C router, set in bridge-modem mode.  I have been able to connect the Zyxel in Routing mode (using the PPPoE user name and password provided by Vodafone Support) to the Voda VDSL2 broadband and get full internet access through it.  With the Zyxel set to bridge-modem mode the RT-AC68U appears to connect to the WAN correctly, using the same PPPoE authentication.  Despite this I cannot access the internet from my PC or other devices connected to the Asus router (though the internal LAN works fine).

The network tools on the Asus successfully 'ping' external servers, so it seems there is a working WAN connection.  I am guessing that the internet access problem is something to do with DNS not working correctly between the Zyxel and Asus, but can't figure out what.  I initially left the Asus to pick up settings automatically from the ISP, including DNS servers.  When that didn't work I tried changing the DNS settings in the LAN page from automatic to fixed (using Google DNS 8.8.8.8) but this didn't help either. 

From reading previous posts on this forum, I know people have been able to get various Asus 'RT' routers working through third-party modems, so I'm obviously doing something wrong!  Any advice or suggestions would be most welcome.  

[Jayach]

You say you are able to ping from the Asus, but can you do so from devices connected to the Asus?

[Psychophile · Accepted solution]

I had another bash at this and eventually solved the problem.  The Windows network diagnostic wizard gave me a clue with a message saying that the 'default gateway could not be reached'.

For some reason I had entered the Zyxel router's default IP address (192.168.1.1) as the Default Gateway address in the Asus DHCP server configs.  But when the Zyxel is in bridge-modem mode it's IP address is not visible on the network, other than by a PC connected to it directly.  So I guess the Asus router was trying to route DNS traffic via this invisible gateway and getting nowhere.  After deleting the Default Gateway setting (and leaving it blank) internet access was restored!

[Anonymous]

At the very least you need to have the local IP of the modem different to the local IP of the router or it's going to cause routing issues. *Depending on the router firmware this may only show up if you attempt to access the modem web-admin.

So it would be common that if your local subnet for the LAN were 192.168.1.0/24 with the router as 192.168.1.1 then the modem would commonly be 192.168.2.1 or 192.168.5.1 with its own (/24) subnet.

*I'm aware that using the router console and the appropriate commands it is possible to direct a local IP to the WAN port but is faff and frankly less robust!

[Batauri]

My default gateway is also blank but I still can't get an internet connection.  Not sure what I'm doing wrong.

Vodafone service: Superfast 2

Router used as modem – Zyxel VMG3925-B10C

IP address changed to 192.168.2.1

802.1q set to 101

To validate Vodafone login details, I switched to routing mode, added the details and got an internet connection Bridge mode selected

Rebooted

Router – AsusRT-AC68U

Updated to latest firmware

Default IP address used: 192.168.1.1

Connected WAN socket – WAN socket of modem

Entered Vodafone login details

Set MTU size to 1492

Rebooted router

...But no connection.  Any suggestions most appreciated

[CrimsonLiar]

Your problem is that you've connected the VGM3925 WAN socket to the WAN socket of the RT-AC68U!

You need to use a LAN socket on the VGM3925 to connect to the WAN of the RT-AC68U.

*Having used this exact same Zyxel modem router in bridged mode, I would have to say that even in bridged mode it buffers everything.  So if you are trying to play games over it you'll need to use QOS or you'll suffer from buffer bloat!

[Batauri]

Many thanks.