jumbo frame testing

Definition:
jumbo: 大型客机（尤指波音 747）, 巨型的；巨大的；特大的

MTU：最大传输单元（Maximum Transmission Unit，MTU）是指一种通信协议的某一层上面所能通过的最大数据报大小（以字节为单位）。最大传输单元这个参数通常与通信接口有关（网络接口卡、串口等）。
jumbo frames: In computer networking, jumbo frames are Ethernet frames with more than 1,500 bytes of payload (MTU). Conventionally, jumbo frames can carry up to 9,000 bytes of payload, but variations exist and some care must be taken when using the term.

 

Requirements for Jumbo Frames:
1.Must be GigLAN backbone (i.e. 1000baseT)
2.Local NICs in the local PCs must support JFs
3.Switches must support JFs

 

Firstly, configure the jumbo frame on endpoints(local and remote machines) by following command:
ifconfig eth1 mtu 9000 (/sbin/ifconfig eth1 mtu 9000)
(MTU will work after above configuration. It's unnecessary to do "service network restart" and could be counterproductive then.)

 

Then, enable jumbo frame on switch(es).
The command "show ports" can show the jumbo frame status on switch.

Followed command can enable/disable jumbo frame:
Console# configure jumbo frame
Console# configure no jumbo frame

 

DELL 5324 should be approximately:
enable
configure
port jumbo-frame
end
copy running-config startup-config
reload

 

Once everything is configured, try pinging with a large packet size and fragmentation disabled. Example:

Ping -s 8500 -M do <destination>

 

IP首部中有一个“不分片标志位(DF)”，路由设备收到一个大于本地MTU的包，要分片的时候，将会检查它，以决定是否分片发送。此机制常常用来做为路径MTU发现的手段。-M参数用于设定此标志.

 

Ping usage:
ping(8) - Linux man page

-M hint
Select Path MTU Discovery strategy. hint may be either do (prohibit fragmentation, even local one), want (do PMTU discovery, fragment locally when packet size is large), or dont (do not set DF flag).

 

[Testing]

Before enable jumbo-frame:
[root@root]# ping -s 2000 -M do 10.10.1.100
PING 10.10.1.100 (10.10.1.100) 2000(2028) bytes of data.

--- 10.10.1.100 ping statistics ---
3 packets transmitted, 0 received, 100% packet loss, time 1999ms

 

After:

[root@root]# ping -s 2000 -M do 10.10.1.100
PING 10.10.1.100 (10.10.1.100) 2000(2028) bytes of data.
2008 bytes from 10.10.1.100: icmp_seq=0 ttl=64 time=0.575 ms
2008 bytes from 10.10.1.100: icmp_seq=1 ttl=64 time=0.195 ms
2008 bytes from 10.10.1.100: icmp_seq=2 ttl=64 time=0.292 ms

[root@root]# ping -s 8000 -M do 10.10.1.100  <----
PING 10.10.1.100 (10.10.1.100) 8000(8028) bytes of data.
8008 bytes from 10.10.1.100: icmp_seq=0 ttl=64 time=1.00 ms
8008 bytes from 10.10.1.100: icmp_seq=1 ttl=64 time=0.596 ms

 

References:
<http://en.wikipedia.org/wiki/Jumbo_frame>
<http://wiki.archlinux.org/index.php/Jumbo_Frames>
<http://docs.us.dell.com/support/edocs/network/5P788/en/ug/pc5224cd.htm>
<http://linux.die.net/man/8/ping>