Configuring MTU on IBM i

MTU AND SKTAP WANs

Due to various layers of encapsulation as well as MTU restriction in both Skytap and Azure, the MTU of network interfaces in IBM i LPARS may need to be adjusted lower than standard 1500 bytes.

CHANGE MTU in IBM i

As the following procedure requires network interfaces to be temporarily disabled, the work should be performed using Skytap SRA, as it allows VM access without depending on networking. Additionally, consider making a template backup of the VM before proceeding.

1) IDENTIFY THE INTERFACE

Run WRKLIND and use option 5 on the ethernet line descriptions present.

You can select multiple line descriptions and page through them if there are multiple NICs present.

Line descriptions in Skytap will be named similar "ETHLINE" or "ETHLINE#" where # is a number

You can compare the local adapter address to the MAC address of the NIC in the Skytap UI to relate a NIC to a line description and resource name in the LPAR.

2) VARY-OFF THE INTERFACE

Run NETSTAT *IFC
Enter option 10 on the interface where the MTU needs to be changed, to down the interface
Enter option  12 on the same interface to work with its line description
Enter option 2 on the line description to vary-off the line and associated resources. Proceed once all 3 rows show VARIED OFF

3) Change MTU

Run CHGTCPIFC LIND(ETHLINE) INTNETADR('IP.ADD.RE.SS') MTU(1240) to set the interface MTU to 1240 bytes, replacing 'IP.ADD.RE.SS' with the actual interface IP, in single-quotes. If the interface is configured with DHCP, specify INTNETADR(*IP4DHCP) instead of an IP address.

4) Vary-on the interface
Run VRYCFG CFGOBJ(ETHLINE) CFGTYPE(*LIN) STATUS(*ON) ASCVRYOFF(*N) to vary on the Line description and run STRTCPIFC INTNETADR('IP.ADD.RE.SS') LIND(ETHLINE) to start the interface (or you can also  take option 1 on the line description to vary it on, then press F3 to return to the interfaces list and start the interface with option 9.) If the interface is configured with DHCP, specify INTNETADR(*IP4DHCP) instead of an IP address.