TFTP error occurs when copying file of C9300 to a TFTP server.

[nishantagrawal0322]

Symptom: TFTP error occurs when we copy running-configuration (or any file in the flash) of C9300-48P to a TFTP server. The configuration file is created on the TFTP server but no actual content in the file (file size=0). The switch display a timed out error in writing the file to the tftp server. Conditions: The issue is noticed when the system MTU is set to 9000 with a TFTP block size of 8192. Workaround: The transfers are successful when the MTU is set to 1500 or the TFTP block size is reduced to 1492 Further Problem Description: After Configuring 'Allow Jumbo Packets' on PC/Server, copy works as expected.

Anybody else facing this?

[MHM Cisco World]

Sorry what issue exactly' are tftp file empty?

[nishantagrawal0322]

Yes

[nishantagrawal0322]

Yes. So here is the situation. We the system MTU is set to 9198 or 9000, and we try to transfer config file from switch to tftp server, it saves the empty file. When the system MTU is set back to default (1500 bytes) tftp config transfer is fine and works as it should.

I have tested this behavior with Catalyst 9300, and catalyst 3650.. 

On other side, I also have Catalyst 3750X, since this switch supports per port jombo frame config, it works on this fine because system mtu is default to 1500 and only the ports that needs jumbo frame are configured to 9000 bytes.

Any clue? is it a bug or normal behavior and was never meant to work.. Please advise.

Thanks

[MHM Cisco World]

make the MTU same 9000 and make the TCP blocksize 1492 <<- only check this.

[nishantagrawal0322]

Tested. Same empty file. The only time it works is when system MTU is set back to 1500 bytes. On another note, if we keep changing MTU size, then the purpose of enabling jumbo frames (MTU 9000) will be defeated, because we need MTU to be set to 9000 in order for it to work with iscsi (VMWare + NetApp)..

Thanks

[MHM Cisco World]

traffic divide into two 
1- traffic pass through SW
2- traffic generate from SW this is TFTP 
here the traffic pass through the SW can be 9000 
the traffic generate from SW as you explain in your post have issue with jumbo size 
so 
let try this way 
you have management interface config MTU 1500 under and use it as source interface toward TFTP, and config the TFTP blockize 1492
this way you will not effect your traffic and in same time you management traffic can work fine ~
try this way

[nishantagrawal0322]

Thanks for your response. But, On 9300,3650, you don't have an option to configure per port MTU. That's the whole issue. If we were able to do so, then the system MTU could stay at 1500 and everything should work as it should. 

[MHM Cisco World · Accepted solution]

Note: You can configure the MTU size for all interfaces on a device at the same time using the global command "system mtu".  Starting with Cisco IOS XE 17.1.1, Catalyst 9000 switches support Per-Port MTU. Per-Port MTU supports port level and port channel level MTU configuration. With Per-Port MTU you can set different MTU values for different interfaces as well as different port channel interfaces.

[nishantagrawal0322]

And I'm running this. 

Switch Ports Model SW Version SW Image Mode
------ ----- ----- ---------- ---------- ----
1 41 C9300-24T 17.06.05 CAT9K_IOSXE INSTALL
* 2 41 C9300-24T 17.06.05 CAT9K_IOSXE INSTALL

And you are absolutely right. I wasn't aware about this update. I can see the option now. 

Is this True for 3650's also?

Please advise.

[ja-robertson]

Hello, FYI I also solved by setting the ip mtu of the vlan interface to 1500

system 9100

int vlan = ip mtu 1500