Solved Does MTU value matter?

[Deleted member 63539]

I found on both Linux and FreeBSD, the default MTU value for my system is 1500. But only on OpenBSD, the MTU is set to 1728. I hardly tell any different in network speed between the three systems, it's almost the same. So does the larger MTU value have any advantages? Thanks.

 

[SirDice]

The default MTU for ethernet is 1500. You can only go higher with gigabit ethernet (they're called "jumbo" frames). You will not notice much, only with certain high traffic like NFS, with bigger frames there's less overhead, which improves performance. But, your switch and everything else will need to support the bigger MTU too (most consumer grade switches don't).

 

[olli@]

It depends on your network environment.

MTU (Maximum Transmission Unit) specifies the maximum size of a network packet (in bytes) that can be transferred at once, without the need for fragmentation. Fragmentation should generally be avoided because it reduces performance, and it might cause problems for certain firewalls.

The standard MTU for Ethernet-based interfaces is 1500. That’s because the standard ethernet frame size is 1518 bytes, and an Ethernet header occupies 18 bytes.

Under certain circumstances, the MTU is reduced, e.g. when PPPoE is used, the MTU is reduced to 1492 because the PPPoE header occupies 8 bytes. Similarly, when using encapsulation or tunneling protocols (IPv6-over-IPv4, DS-Lite etc.), the MTU is reduced accordingly in order to accommodate for the protocol overhead.

On the other hand, the MTU can be increased if all network components involved support so-called “jumbo frames”. A typical MTU setting for jumbo frames would be 9000, some newer 10GbE+ adapters even support 16 KB. This can improve the performance of local file servers, for example.

The value of 1728 that you mentioned is quite unusual. I’ve never seen this before. It’s too small to be useful as jumbo frames, but it’s too large for standard Ethernet (so it’ll cause fragmentation). What kind of network interface is this, exactly? Could it be a WiFi interface? I seem to remember that the standard MTU of WiFi is somewhat larger than Ethernet, but I don’t think it’s 1728. This number doesn’t really ring a bell.