Support Support Downloads Knowledge Base Case Manager My Juniper Community

Knowledge Base

Search our Knowledge Base sites to find answers to your questions.

Ask All Knowledge Base Sites All Knowledge Base Sites JunosE Defect (KA)Knowledge BaseSecurity AdvisoriesTechnical BulletinsTechnotes Sign in to display secure content and recently viewed articles

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

0

0

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

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.

 

Symptoms:

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
^C
--- x.x.x.x ping statistics ---

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

 

Solution:

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

 

Related Links

Comment on this article > Affected Products Browse the Knowledge Base for more articles related to these product categories. Select a category to begin.

Getting Up and Running with Junos

Getting Up and Running with Junos Security Alerts and Vulnerabilities Product Alerts and Software Release Notices Problem Report (PR) Search Tool EOL Notices and Bulletins JTAC User Guide Customer Care User Guide Pathfinder SRX High Availability Configurator SRX VPN Configurator Training Courses and Videos End User Licence Agreement Global Search