Hello!
(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!
Custom MTU with PPPoE bug (1.3.15)
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Custom MTU with PPPoE bug (1.3.15)
Last edited by Lrdx on Sun Jun 19, 2011 7:11 pm, edited 1 time in total.
Re: Custom MTU with PPPoE (1.3.13)
Update:
I have just installed 1.3.15, and it still has this bug. Turning off custom MTU, or setting it to 1500 on the web interface only sets the MTU for pppoe-wan to the default 1492, but for eth0.1 it does not get set back to 1500, it stays on the last value it was set to.
I have just installed 1.3.15, and it still has this bug. Turning off custom MTU, or setting it to 1500 on the web interface only sets the MTU for pppoe-wan to the default 1492, but for eth0.1 it does not get set back to 1500, it stays on the last value it was set to.
Re: Custom MTU with PPPoE bug (1.3.15)
I'm hesitant to play with this too much right now. I agree that it's a bug, but I'm a bit worried that I could make things worse by trying to introduce a solution to this issue right now. I'm pretty sure the issue is in the OpenWrt code -- leaving the mtu setting blank doesn't force the default unless you reboot, so when the mtu setting gets reverted, the default doesn't get set. Fixing it means either (1) always setting MTU explicitly (which I'm worried could have some bad side effects, especially if a bug gets introduced in this and would need significant testing on multiple platforms) or (2) patching upstream openwrt code which can get really ugly and has the same problem of potentially unwanted side-effects.
Basically, I'm worried that doing what it would take to fix this right now has a small but not insignificant chance of temporarily making things a lot worse.
Therefore I'm going to wait to address this bug until after the stable release. I can see why this is annoying, but not exactly critical -- you can always set the MTU explicitly, to avoid the problem, right?
Basically, I'm worried that doing what it would take to fix this right now has a small but not insignificant chance of temporarily making things a lot worse.
Therefore I'm going to wait to address this bug until after the stable release. I can see why this is annoying, but not exactly critical -- you can always set the MTU explicitly, to avoid the problem, right?
Re: Custom MTU with PPPoE bug (1.3.15)
It's not critical, I can always open a terminal and write or something similar to fix it.
Code: Select all
ifconfig eth0.1 mtu 1500