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[ScreenOS] How to enable path MTU?

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Article ID: KB8021 KB Last Updated: 18 Dec 2017Version: 4.0
Summary:
Path MTU can be enabled on device level or interface level in ScreenOS 5.3 and above.
Symptoms:
Hosts that support Path-MTU discovery need to receive ICMP packet too big messages (type 3 code 4) from the devices in the path in order to be able to determine the lowest MTU in the path. Juniper firewalls do not send ICMP type 3 code 4 messages by default; it must be configured on the firewall.
Solution:
In Screen OS 5.3 or higher, it is possible to send ICMP type 3 code 4 message back to the source when the received packet has the DF bit set and the packet size is larger than the MTU size configured on the egress interface.

This feature is called Path MTU and can be enabled in either of the following two ways:

1.  Device level:
set flow path-mtu  
This command enables the feature in the device level.

2.  Interface level:
set interface <name> pmtu ipv4
This command enables Path MTU only in the interface level.
Modification History:
2017-12-07: Article reviewed for accuracy. No changes made. Article is correct and complete.
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