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To improve network performance jumbo frames are often used. However, if the settings are off then network performance will suffer. This article will explain how to use ping to determine if all nodes and network gear are using the correct MTU settings. Determine MTU setting Before testing can begin the MTU size needs to be determined.
On Windows the netsh command can be used:
C:\> netsh int ip show int
Idx Met MTU State Name
--------------------------------------------------------------
1 50 4294967295 connected Loopback Pseudo-Interface 1
10 10 1500 connected Local Area Connection
On Linux the ifconfig command can be used:
# ifconfig
eth0 Link encap:Ethernet HWaddr 68:05:CA:03:03:88
inet addr:192.168.105.170 Bcast:192.168.105.255 Mask:255.255.255.0
inet6 addr: fe80::6a05:caff:fe03:388/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:33171101471 errors:165 dropped:0 overruns:0 frame:165
TX packets:36797914598 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:3198464811000 (2.9 TiB) TX bytes:22640130397766 (20.5 TiB)
Interrupt:16 Memory:f7ec0000-f7ee0000
Testing from Linux
On Linux, the -M, -s and -c flags will be used. The -M tells ping to NOT fragment, the -s sets the packet size and -c sets the number of pings to send. Assuming a MTU setting of 9000, subtract 28 to get the packet size to use, the command will be:
ping REMOTE_HOSTNAME -c 10 -M do -s 8972
If the MTU sizes don’t match the ping command will return “Packet needs to be fragmented by DF set”.
[root@u12 log]# ping -c 10 -M do -s 8972 t10
PING t10.citrusleaf.local (192.168.105.80) 8972(8820) bytes of data.
From u12.citrusleaf.local (192.168.120.112) icmp_seq=1 Frag needed and DF set (mtu = 1500)
From u12.citrusleaf.local (192.168.120.112) icmp_seq=1 Frag needed and DF set (mtu = 1500)
From u12.citrusleaf.local (192.168.120.112) icmp_seq=1 Frag needed and DF set (mtu = 1500)
From u12.citrusleaf.local (192.168.120.112) icmp_seq=1 Frag needed and DF set (mtu = 1500)
From u12.citrusleaf.local (192.168.120.112) icmp_seq=1 Frag needed and DF set (mtu = 1500)
From u12.citrusleaf.local (192.168.120.112) icmp_seq=1 Frag needed and DF set (mtu = 1500)
From u12.citrusleaf.local (192.168.120.112) icmp_seq=1 Frag needed and DF set (mtu = 1500)
From u12.citrusleaf.local (192.168.120.112) icmp_seq=1 Frag needed and DF set (mtu = 1500)
Testing from Windows
On Windows the flags -f and -l will be used; -f flag tells ping to NOT fragment the packets and the -l sets the packets size. Assuming a MTU setting of 9000, subtract 28 to get the packet size to use, the command will be:
ping REMOTE_HOSTNAME -f -l 8972
If MTU sizes don’t match you will get “Packet needs to be fragmented by DF set” instead of the normal ping output.
C:\Users\citrusleaf>ping t1 -f -l 8972
Pinging t1.citrusleaf.local [192.168.105.71] with 8972 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Ping statistics for 192.168.105.71:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),