Just as we discussed earlier. I take pppoe-wan interface and specify one MTU in Interface -> wan -> Override MTU and another (different) MTU in Devices -> pppoe-wan.
Yes, we discussed earlier, but I [still] don't understand your point or why you feel a setting "overrides" another.
The PPP is going over Ethernet. It seems pretty straightforward and obvious that the raw Ethernet PHY would need a larger MTU than the protocol you're using over it (i.e. PPP). So it's really unclear what you're asking.
Since the OP has a dsl interface, it's moot anyway.
But the apply to the same pppoe-wan interface! That's the point!
My apologies. I guess you forgot there's an 8 byte overhead to carry PPP over Ethernet, that's what we've been discussing all along.
This is why I asked for your actual config and not a screenshot:
Here, you set 1492 - but it's unclear what interface this is. It appears we're not referring to the same section.
You then show this.
As we discussed, it's unclear what interface you're configuring.
If you're merely noting that a poster said the ppp interface makes the x -8 calculation - then simply account for that. Nonetheless the RAW interface needs to be 8 larger than the PPPoE being carried over it.
This is exactly why I'm making it smaller than 1500. Which is the MTU for eth0, the undelying device for pppoe-wan. All those settings are shown on my screenshots.
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OK, cool. We're on the same page. I acknowledge that the other poster stated PPPoE automatically calculates the overhead on OpenWrt. So if (e.g.) an ISP does offer Jumbo Frames for a full 1500 PPPoE, one would add, add 8 to both the raw and PPP interfaces setting.
As you seem to agree - 1508 for both (and PPPoE will subtract 8 to == 1500).
Correct?
Correct. But my question was about having two places to specify MTU for the same interface. In my case both of them seemingly should apply to pppoe-wan, not underlying eth0 (anyway, it changes nothing for eth0 too).
They're not the same interface, they're two - just as much as a VPN interface has an MTU setting and must use an IP-based interface. It's a point-to-point tunnel over an Ethernet connection. In this case, it's just a tunnel on Layer 2 (over raw Ethernet to the ISP-side), in contrast, most VPNs tunnel Layer 3 (over IP) to a server on the Internet).
If I understand correctly, changing PPPoE to 1508 didn't change eth0 - this is correct?
If so, in our Jumbo Frame example, one would need to configure the raw interface, eth0 to 1508 as well.
Example - what if the ISP offered one service on normal Ethernet, and another on the PPPoE (i.e. connection to a credit card processor, etc.)?
Then how could you explain that they both are being set to the SAME value received from ISP? Of course if everything is in auto mode.
There's no such automatic MTU setting for physical Ethernet, it must be manually set on both sides. The Global standard default just happens to be 1500. This is another premise I was unclear why you were implying.
config interface 'wan'
option proto 'pppoe'
option device 'eth0'
...
option mtu '1492'
config device
option name 'pppoe-wan'
option mtu '1492'
Only one of these settings will apply. The one in 'device' section. Another one will change nothing even I remove 'device' section.
Auto mode is when you specify nothing, rely on defaults.
You haven't configured:
In your case you still need to make a config device section for your eth0 to add MTU.
That's why it remains 1500.
This is why I asked you to verify if you were on the interface or device page in LuCI.
So once again, how those two are different? Let's assume you're right then why those two set to the same value received from ISP by default?
I don't need to increase it... why would I?
I donno what you're asking again. You showed an invalid UCI interface config with a bogus MTU - and asked the difference?
I highlighted you set MTU on the device page/section.
Then I assume this is a moot discussion.
Let's narrow it down. Tell me what interface MTU will it change if everything else was left at its defaults and this MTU is specified?
I showed no UCI configs, just two LUCI screenshots and config file. What makes it invalid? I do not overshoot underlying interface MTU set by default...
Either nothing, because it's either invalid or it's the PPPoE. But again, you have it set at PPPoE, and I assume the PPPoE interface. You were providing that information, I explained how to set the Ethernet interface - or "device" in OpenWrt terminology.
One would need to set:
config device
option name 'eth0'
option mtu '1600'
To alter the raw eth0 device.
Your config interface stanzas only create the PPPoE interface on device eth0.
Edit:
This is done on the Devices tab, not the Interfaces tab.
What does this mean? Another secret MTU no one sees?