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Cisco Jumbo frame setting


Here is a little tidbit to remember. I was configuring a switch for a new iSCSI SAN install. To make it easier I was copying configuration settings from an existing SAN switch when I noticed there was nothing setting the Jumbo frames to 9000. I verified everything was ok on the production network by using ping, setting the size, and telling it not to fragment. I then used a show system MTU command and the switch shows Jumbo MTU set to 9000. I little searching online and I found that Cisco switches don’t store the Jumbo frame setting in the configuration file. Here is the quote from the Cisco documentation:

The system MTU setting is saved in the switch environmental variable in NVRAM and becomes effective when the switch reloads. Unlike the system MTU routing configuration, the MTU settings you enter with the system mtu and system mtu jumbo commands are not saved in the switch Cisco IOS configuration file, even if you enter the copy running-config startup-config privileged EXEC command. Therefore, if you use TFTP to configure a new switch by using a backup configuration file and want the system MTU to be other than the default, you must explicitly configure the system mtu and system mtu jumbo settings on the new switch and then reload the switch.

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