sinshiva
Very Senior Member
Jumbo frames [Bug?]
So, i think i understand why jumbo frames are a pita. to use them effectively, aside from the fact that the devices that you intend to interact with eachother require the same jumbo frame support to achieve the higher MTU usage, you want the switch to be able to assign the bridge the higher MTU. however, for whatever reason if the bridge isn't the MTU for an internet connection like DSL, it's like the bridge takes the higher mtu packet and tries to pass it to the wan interface, which tells the bridge 'no', but that doesn't get passed on to the clients of the bridge or is dropped entirely.
So, if you want to use jumbo frames and you have DSL, you really need one router with the MTU on both the WAN and br0 set appropriately connected to another router/switch with the bridge set at the jumbo frame MTU. This way the devices will talk to eachother in jumbo frames and through the internet at your dsl MTU. This would probably be a good opportunity to make use of LACP.
[edit]
with cable or any isp running a standard mtu of 1500, the default wan/br0 mtu would obviously be fine and you'd just connect the devices using jumbo frames to an attached switch[/edit]
[edit2/]
it occurs to me that with jumbo frames enabled, the devices should be able to ignore the br0 MTU when talking to eachother, but when they want to talk directly to the router or through the internet, they must utilize the br0 MTU. I only have one gigabit device at home right now, so i can't test, but i think that if you are on DSL with less than 1500 MTU, the way things are supposed to work is that you'd set the WAN MTU to whatever, like 1456, which should then automatically update the rest of the interfaces to the same MTU, particularly br0. Then, with jumbo frames enabled the switch should forward the jumbo packets between jumbo devices.
With DSL and asuswrt, if you don't set this up via init-start scripts atm, you end up with a WAN MTU set correctly, a br0 stuck at 1500 and so devices with higher MTU's might be able to talk to eachother and the immediate router alright, but you can end up with errors trying to traverse the internet if your WAN MTU is smaller than the br0 MTU.
so yea, i'm pretty sure that this is a bug?
So, i think i understand why jumbo frames are a pita. to use them effectively, aside from the fact that the devices that you intend to interact with eachother require the same jumbo frame support to achieve the higher MTU usage, you want the switch to be able to assign the bridge the higher MTU. however, for whatever reason if the bridge isn't the MTU for an internet connection like DSL, it's like the bridge takes the higher mtu packet and tries to pass it to the wan interface, which tells the bridge 'no', but that doesn't get passed on to the clients of the bridge or is dropped entirely.
So, if you want to use jumbo frames and you have DSL, you really need one router with the MTU on both the WAN and br0 set appropriately connected to another router/switch with the bridge set at the jumbo frame MTU. This way the devices will talk to eachother in jumbo frames and through the internet at your dsl MTU. This would probably be a good opportunity to make use of LACP.
[edit]
with cable or any isp running a standard mtu of 1500, the default wan/br0 mtu would obviously be fine and you'd just connect the devices using jumbo frames to an attached switch[/edit]
[edit2/]
it occurs to me that with jumbo frames enabled, the devices should be able to ignore the br0 MTU when talking to eachother, but when they want to talk directly to the router or through the internet, they must utilize the br0 MTU. I only have one gigabit device at home right now, so i can't test, but i think that if you are on DSL with less than 1500 MTU, the way things are supposed to work is that you'd set the WAN MTU to whatever, like 1456, which should then automatically update the rest of the interfaces to the same MTU, particularly br0. Then, with jumbo frames enabled the switch should forward the jumbo packets between jumbo devices.
With DSL and asuswrt, if you don't set this up via init-start scripts atm, you end up with a WAN MTU set correctly, a br0 stuck at 1500 and so devices with higher MTU's might be able to talk to eachother and the immediate router alright, but you can end up with errors trying to traverse the internet if your WAN MTU is smaller than the br0 MTU.
so yea, i'm pretty sure that this is a bug?
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