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[M/MX/T/TX] Cannot ping with payload greater than 1472 when do-not-fragment is chosen and interface has default MTU settings



Article ID: KB28299 KB Last Updated: 09 Apr 2020Version: 2.0

This article explains why you cannot have a ping payload larger than 1472 when you designate do-not-fragment on Ethernet interfaces with default MTU settings.



When trying to ping from an Ethernet interface with default MTU settings, if you try to set a payload greater than 1472 and do-not-fragment, you may see something like this:

router@lab# run ping x.x.x.x size 1500 do-not-fragment

PING x.x.x.x (x.x.x.x): 1500 data bytes
ping: sendto: Message too long
ping: sendto: Message too long
ping: sendto: Message too long
ping: sendto: Message too long
--- x.x.x.x ping statistics ---

4 packets transmitted, 0 packets received, 100% packet loss



The reason for this is that by default, Juniper uses the standard IP MTU of 1500. Along with the default size, you have to account for 8 bytes being used for the ICMP header and another 20 bytes for the IP address.

This comes to:

1500 - 8 - 20 = 1472

As such, 1472 bytes is the largest payload that you can set where you have "do fragment" turned off. 


Modification History:

2020-04-09: Article checked for accuracy; found to be valid and relevant; minor non-technical modifications made


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