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FAQ-What do the -l and -f options of the ping command mean ,How can the MTU be set

Publication Date:  2012-07-25 Views:  113 Downloads:  0

Issue Description

Jumbo frames are common on networks. How can I enable the OptiX equipment to transfer jumbo frames by using the ping command? 

Alarm Information

The remote monitor (RMON) monitors the statistics on all the data frames that have been transmitted and received. 

Handling Process

Set the MTU on a network interface as follows:
1. Choose Start > Run. Type regedit and then click OK in the Run dialog box.
2. Go to the following registry key:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\<ID for network interface>
3. Choose Edit > New > DWORD Value.
4. Type MTU and then press ENTER.
5. Choose Edit > Modify.
6. Type an MTU size in the Value Data box, and then click OK.
7. Quit Registry Editor, and then restart the PC. 

Root Cause

As a common troubleshooting tool, the ping command uses Layer 3 Internet Control Message Protocol (ICMP). Certain options of the ping command help test the support of equipment for jumbo frames.
The -l and -f options of the ping command are as follows:
The -l option sends buffer size, that is, the payload size.
The default maximum transmission unit (MTU) size for the Windows operating system is 1,500 bytes. If a packet exceeds 1,500 bytes, the packet must be fragmented.
? Assume that the -l option is set to 1,472. The 1,472 bytes plus the 20 byte for the IP header and the 8 bytes for the ICMP header equals the 1,500 bytes.
? Assume that the -l option is set to a value greater than 1,472, such as 1,473 and that packet fragmentation is disabled. The ping command reports errors as follows because the frame length exceeds the MTU size (1,500 bytes):
C:\Documents and Settings\user>ping -l 1473 �f
Pinging with 1473 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.
Therefore, the ping command limits IP packets to the MTU size. If the MTU size is set to the default of 1,500 bytes, a message received by the OptiX equipment contains up to 1,518 bytes. Of the 1,518 bytes, 1,500 bytes are for the IP packet and the remaining 18 bytes are for the MAC header.
The -f option sets the Don’t Fragment flag in packet. That is, the option helps disable packet fragmentation. 


The attachment describes the ET1 test report that this case summarizes. The test report helps you understand how the data board processes packets.