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

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Article ID: KB5378 KB Last Updated: 21 Dec 2017Version: 6.0
Summary:
Although the MTU size is hard coded on the NetScreen-5000 and ISG-2000, the MSS value can be configured.
 
Symptoms:
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.
Solution:
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.

NOTES:

  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 https://kb.juniper.net/KB12843  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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