Jumbo Frame Information

A standard ethernet frame can carry a 1500 byte payload. This is normally the value quoted for ethernet's MTU (Maximum Transmission Unit) and the value you see e.g. from Unix ifconfig commands. Sometimes people will include 14 byte ethernet headers, four byte 802.1q VLAN tags, and 4 byte CRC's in their counts, for 1514, 1518, 1522. 1518 is the most common number. Juniper uses 1514 (no CRC). Cisco allows up to 1548 bytes in Interlink Switch frames. There are many reasons your MTU might be less than 1500 including:

"Jumbo frames" often mean 9000 bytes for Gigabit ethernet, but can refer to anything over 1500 bytes. MTUs seen "in the wild" include: 9192, 9180, 9176, 9174, 4470. A survey of Internet2 connectors in Feb 2003 showed 66% at 4470, 27% at 9180 and above, 8% between 9000-9176.

Many pings will let you do MTU discovery with the -M option:

 damp-navo$ ping -s 8000 -d -v -M do damp-asc2
 PING damp-asc2-ge (138.18.22.5) from 204.222.177.220 : 8000(8028) bytes of data.
 From so-0_0_0.wpafb.dren.net (138.18.1.5) icmp_seq=1 Frag needed and DF set (mtu = 4352)
 8008 bytes from damp-asc2-ge (138.18.22.5): icmp_seq=1 ttl=60 time=47.6 ms
 ping: local error: Message too long, mtu=4352

Tracepath is also useful for MTU checking:

 damp-navo$ tracepath damp-asc2
 1?: [LOCALHOST]     pmtu 9000
 1:  204.222.177.222 (204.222.177.222)                      0.892ms
 2:  204.222.177.113 (204.222.177.113)                      0.333ms
 3:  ge-0_1_0.navo.dren.net (138.18.142.1)                asymm  4   1.188ms
 4:  so-0_0_0.wpafb.dren.net (138.18.1.5)                 asymm  5  47.197ms
 5:  so-0_0_0.wpafb.dren.net (138.18.1.5)                  46.646ms pmtu 8166
 5:  damp-asc2-ge (138.18.22.5)                            94.564ms reached
     Resume: pmtu 8166 hops 5 back 5

See also a traceroute with path MTU discovery (-M).

On DREN, settings include: