In message <5FC2EE64-3EA0-11D9-A37D-000D93B24C7A(a)isc.org>, Joe Abley writes:

On 24 Nov 2004, at 22:11, admin wrote:

...

no, orcon told me to use 250 and it seems 1514 is the default in linux.

The MTU usually depends on the frame size that can be accommodated by an interface. There is no useful "default" MTU. I seem to remember hearing from someone that Linux includes the encapsulating frame headers in its calculations when it interprets an MTU, which would make 1514 make sense for Ethernet. Kind of.

That's the first I've heard of that. All the Linux boxes that I administer have a MTU of 1500 (by default, anyway) on their Ethernet interfaces.[0] To the best of my knowledge the MTU figure given to Linux network tools is the same as the MTU figure given to any other OS (ie excluding link layer framing overhead). Some googling turns up several references to "mtu 1514", from various equipment (eg, Cisco loopbacks with a MTU of 1514's, tunnels on various things, PPP interfaces, etc), but it's not clear what made someone consider it a "default".[1][2] There are also various reports of 1514 octet MTUs on ADSL connections (presumably on the interface facing the DSLAM), with various suggestions to change them to other things, including some elaborate "optimisations" based PPPoA or PPPoE overhead and ATM cell tax. 14 octets extra doesn't seem to match PPPoE or PPPoA overhead though.

However, in general, if you reduce your MTU below 1500 bytes you can expect all the inexpertly-firewalled, pMTUd-enabled servers of the world to stop sending you packets, sooner or later.

Ah, the joys of "I don't know what ICMP is, it sounds dangerous, we'd better block it" firewall administration. Ewen [0] The default MTU on some of the other interface types is different, but a lot of them seem to default to 1500 for Ethernet compatiblity. [1] There are a bunch of people "learnedly" pronoucing that the maximum Ethernet MTU is 1514 but they appear to be deluded; about the only thing which makes some sense is that capturing packets off an Ethernet network with link headers will give you 1514 octet frames (since you don't capture the CRC at the end of the frame), which seems to confuse some people. [2] PPP on a Linux system I tried defaults to 1500 octets, presumably to deal with packets-of-ethernet without fragmentation; but it appears there may be some Linux distributions which ship with PPP defaulting MTU to something else.