I have a 450 working fine with dual ADSL lines (two wans):
- Ether 1: pppoe client (conected to Telefonica router in briged mode)
- Ether 2: static ip (conected to Telefonica router in NAT mode)
All traffic go to Ether1 (by interface pppoe client), except http traffic (exists a route for this) that use Ether2
This system works fine for months (and now) over ADSL.
But the problem is, I install this system in dual VDSL (NOT ADSL), and I see that Ether1(pppoe client) don’t works fine.
Ether2 works fine, but Ether1 not, for example, I delete the route for http traffic to Ether2, and I see that I can see http://www.google.es but not others webs. Messenger also fails, however bittorrent, skype, etc. works fine.
Why it works with some pages and programs only?
I think that is a MTU problem, and I try other values (1300 to 1500), but nothing.
Any idea?? 
Notes.-
- The VDSL connection works fine directly over a PC.
- The 450 works fine over ADSL. 450 over VDSL dont work
- I try switch both telephone VDSL lines, and is the same.
Exports:
/interface pppoe-client
add ac-name=“” add-default-route=yes allow=pap,chap,mschap1,mschap2 dial-on-demand=yes disabled=no interface=ether1 max-mru=1480 max-mtu=1480 mrru=disabled name=pppoe-out1 password=adslppp profile=default service-name=“” use-peer-dns=yes user=
adslppp@telefonicanetpa
Mangle at end:
One dynamic forward rule for reduce MSS of incoming interface pppoe connections to 1440
One dynamic forward rule for reduce MSS of outgoing interface pppoe connections to 1452