My tunnel 6in4 works for few seconds and stops

[juanignacioquesada]

I configured my tunnel on my Asus router. As soon as I hit apply, which reset the ip stack, I can start pinging over ip6. But after few seconds the whole thing stops working. I am running the test right from the router itself.
My tunnel is defined in tunnelbroker like this:
IPv6 Tunnel Endpoints
Server IPv4 Address:XXX.XXX.XXX.XXX
Server IPv6 Address:2001:470:X:XXX::1/64
Client IPv4 Address:X.X.X.X
Client IPv6 Address:2001:470:X:XXX::2/64
Routed IPv6 Prefixes
Routed /64:2001:470:X:XXX::/64
Routed /48:2001:470:ZZXXZ::/48 [X]

On the router I have:
Connection Type: 6in4
Server IPv4: XXX.XXX.XXX.XXX
Client IPv6: 2001:470:X:XXX::2
Ipv6 Prefix Length: 64
Server IPv6: 2001:470:X:XXX::1
MTU: 1480
TTL: 255

LAN Prefix Length: 64
LAN IPv6 Prefix: 2001:470:X:XXX::

I tried LAN Prefix Length 48 with Prefix 2001:470:XXX:: with the exact same results.

Can anyone help me understanding what I may be doing wrong?

[cholzhauer]

Thank you for not obfuscating the IPs.

Which Asus router?  Up to date firmware?

I'm also running an ASUS router and have had no issues.  Is there another firewall upstream? 

[juanignacioquesada]

Crap! Dumb me!
I am using AC88U with Merlin WRT 384.10.2.

[juanignacioquesada]

Which Asus router?  Up to date firmware?

I'm also running an ASUS router and have had no issues.  Is there another firewall upstream?

Forgot to mention. No other FW upstream. Strange is I can ping6 for few secs and then it just stops

[cholzhauer]

So not the stock ASUS firmware.

Let's rule something out...if you put the tunnel on different hardware, do you have the same issues?

[juanignacioquesada]

So not the stock ASUS firmware.

Let's rule something out...if you put the tunnel on different hardware, do you have the same issues?

I've just configured the tunnel directly on my MacOS and it seems to work with no issues

[cholzhauer]

So it seems like an OS/device thing.  Is there a newer version of the software you're running on the router?

[juanignacioquesada]

No, I'm running the latest firmware. Unlike DD-WRT or Tomato, Merlin-WRT is actually supported by Asus

[juanignacioquesada]

I dropped MTU from 1480 to 1472 and it is a bit more stable now. I'll check more

[juanignacioquesada]

It looks like dropping down the MTU to 1472 did the trick. It's now working solid