How do I change the MTU on my bridge on the CRS317?
Never had this issue before.
You can see every single interface has a higher MTU than the bridge interface which does not allow me to change manually..
Change it to what?
How about reading some docs about ethernet frame sizes? Start with Wikipedia article. IP datagram max size is 65535 bytes (and this is not referred to as MTU).mkx, the MTU of an ethernet frame is 1500 Byte.. The MTU of an IP Packet is 65535 Byte.
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However the actual MTU always remains 1500 Byte..
Right ... by an L3 device, such as router. Not by L2 device, such as switch or bridge.So whatever happens the packet will be fragmented to 1500 Byte.. Right ?
As I wrote: while bridge had l2mtu of 65535, I set mtu to 10000 ... after I added wlan interface with l2mtu of 1600, bridge l2mtu reduced to 1592 and actual-mtu shrunk to 1500 ... but mtu property remained 10000. And documentation states that actual-mtu and l2mtu are read-only ...I dont understand what that "actual MTU" refers to....!!!
What?Byte..
How about reading some docs about ethernet frame sizes? Start with Wikipedia article. IP datagram max size is 65535 bytes (and this is not referred to as MTU).
Yes, i've posted an exhaustive list of interfaces, that is the first thing i checked and double checked.
So it seems that l2mtu of a bridge gets set to smallest l2mtu value of all slave interfaces (OP should check the whole list of interfaces, from the screenshot it's not clear if the whole list is shown or it's actually truncated) and can not be set. And it's logical that MTU of bridge is set to smallest MTU value of slave interfaces ... bridge (the switch-like entity, being L2) doesn't perform fragmentation (only L3 device can do that), so bridge interface (which is actually used by router to interact with that L2 domain, it's not used when "only" switching traffic between slave interfaces) should have MTU small enough to pass whole ethernet frame via any of its slave interfaces ... because the egress physical port for a frame gets known only after frame lands on the bridge (the switch-like entity) consulting ARP tables.
... over given medium ... which is L2, in our case ethernet with its limitation, whatever it might be (perhaps standard 1500 bytes plus overhead or jumbo with over 10kB frame sizes). I've yet to see transport technology (L2) with support for 64kB frames.In the context of Internet Protocol, MTU refers to the maximum size of an IP packet that can be transmitted without fragmentation over a given medium.
Also the IPv4 MTU is 64 kib which is 65535 Byte...
I think the reason, specially on WISP-oriented gear, not to use jumbo frames by default, is due to radios not being able to pass L2 jumbo frames at all... you have to use the minimum commonly supported one and use it throughout the network/segment to avoid fragmentation.And my speculation about why vendors don't support huge MTUs just on all of their devices: if L2 device (switch) is a store&forward device (most of them are), then large configurable MTU sizes mean larger RAM (used as frame cache) is needed ... rising price of device (every penny counts) ... for a feature which is rarely used anyways.