(If a topic with a similar subject already exists, sorry, I got a "The following words in your search query were ignored because they are too common words: mtu pppoe." (or similar) error while searching.. I'd never thought mtu and pppoe are common words.
)I have found two issues issue with the "custom MTU" setting: First, the same custom MTU is set for the VLAN and the PPPoE interfaces, and second, it does not revert back correctly to 1500 when I turn the "custom-ness" off.
In a bit detail: I use an ASUS WL-500GP (v1) with a PPPoE ADSL so my wan interfaces at the router are eth0.1 and pppoe-wan. The default configuration is that eth0.1 has an MTU of 1500, and pppoe-wan has 1492 (checked with ifconfig in a terminal session). If I set a custom MTU of (for instance) 1454 on the web interface, both eth0.1 and pppoe-wan gets 1454. This is an error, as if I have a PPP packet of 1454 bytes, it will be 1462 bytes on Ethernet.. I'd leave the eth0.1 MTU on 1500 bytes in this case, as the actual WAN interface is only the PPPoE connection. The "pure" VLAN interface can be used to access the DSL modem's webif/SSH/terminal, and I see no point in limiting the packet size to lower than 1500 for that.
Second issue is about clearing this setting: If I untick the "use custom MTU" tickbox, the pppoe-wan's MTU correctly gets reverted to 1492, but the MTU of the eth0.1 will stay on the previously-set 1454. This should been reverted to the default as well. As the webif tries to be smart and turns off the "use custom" feature when I write 1500, there is actually no way of setting the MTU of eth0.1 back to 1500, at least not until I completely redo the WAN configuration from scratch! ( I think setting the WAN type to DHCP and back to PPPoE helped. )
I have this issue on Gargoyle 1.3.13. I had no way of trying 1.3.14 yet, as I currently cannot bring router offline to update the configurations. Is this fixed in 1.3.14? I did not see that in the change log, so I guess it is not, so please take a look on it.
Thank you!