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[ScreenOS] Cannot change interface MTU size on the NetScreen-5000 and ISG Series



Article ID: KB5378 KB Last Updated: 21 Dec 2017Version: 6.0
Although the MTU size is hard coded on the NetScreen-5000 and ISG-2000, the MSS value can be configured.
Where do you change the MTU size for interfaces on the NS-5000 and ISG devices?
The interface MTU size is configurable only on SSG devices.
The MTU size on the NetScreen-5000, NetScreen-ISG-2000 is hard-coded to 1500 bytes, and it cannot be modified in the WebUI or CLI.

The MSS value can be modified. The parameter for setting this is as follows:

set flow tcp-mss <mss value>                (for VPN TCP traffic)
set flow all-tcp-mss <mss value>            (for clear TCP traffic)

Typically, you want to use a MSS value low enough so that the effective packet size falls below the MTU.  See KB7952 for an explanation of these two commands.


  1. These 'set flow' commands affect TCP packets only and will not affect the datagram size generated if the sender protocol is UDP.   The command set flow path-mtu is also highly recommended.
  2. Inability to modify interface MTU is not the same as Jumbo frame support. Jumbo frames ae still supported on certain firmware-interface-feature combinations of ISGs and NS-5000 boxes. Please refer to  for details.


Modification History:
‚Äč2017-12-07: Article reviewed for accuracy. Removed references of very old and End of life products. Article is correct and complete.

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