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Old 10-06-2015, 07:32 PM   #1
pcCoder
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MTU size and 802.1q


When configuring VLAN interfaces, they show the same MTU as the parent interface in the output of ifconfig, ip link show, etc. Will this cause a problem if a payload the full size (1500) is sent out the VLAN interface, or does it automatically account for that and the parent interface MTU only apply to it directly?

If it does cause a problem, then I assume I need to adjust the main interface MTU to 1504. But if I do that, is there a way I can tell the kernel to allow packet sizes with a payload of 1504 for the main interface from the VLAN interfaces, but still only allow 1500 for the packets sent directly via the main interface, essentially to ensure it doesn't exceed the MTU of access ports on the connected switch. The switch doesn't support jumbo frames and only has an MTU of 1500, but will allow the extra 4 bytes on tagged ports, so I want to make sure that traffic sent directly via the main interface untagged will not try to consume the extra 4 bytes if I set the MTU to 1504, and then have a problem at the switch.
 
Old 10-07-2015, 07:37 AM   #2
lazydog
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You should start HERE
 
Old 10-07-2015, 09:55 AM   #3
pcCoder
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Some of what I've read mentions layer 2 MTU vs layer 3 MTU. Basically I want the layer 3 MTU to remain the same on all interfaces (1500 bytes), but the layer 2 MTU for the physical interface to be able to handle the entire frame whether it comes from the untagged interface itself (1518 bytes) or from a VLAN interface (1522 bytes). Is there a command that will specifically set the driver/hardware layer 2 MTU while leaving the layer 3 MTU alone? Or if I set the MTU of the main interface to 1504, do I have to worry about a situation where the untagged traffic with a payload of 1504 bytes will be received just fine on the trunk port of the connected switch, but will be dropped because the access ports on the switch only support 1500?
 
Old 10-10-2015, 10:41 PM   #4
lazydog
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Best advice I can give you is do not mess with the MTU unless you know what you are doing. I've seen people who think they know what they are doing bring networks to a crawl because they though they had to mess with the MTU. You do not have to mess with the MTU unless you have a specific reason to and then you would know what you are doing hopefully.
 
Old 12-24-2015, 03:39 PM   #5
pcCoder
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I agree. This is all just hypothetical with an interest in understanding the underlying operations better. For instance, how is it that a VLAN interface and the main interface can have the same MTU? if eth0.10 has an MTU of 1500, and eth0 has an MTU of 1500, wouldn't that cause a problem if traffic is transmitted on eth0.10 with the full 1500 byte payload, but due to tagging becomes 1504 bytes at eth0? And the same if QinQ was used with something like eth0.10.20?
 
  


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