System MTU 3850, 9300 Switches

[igdston.cisco]

In previous catalyst switches, i.e., 2960, 3560, 3750, etc., when needing to configure Jumbo Frames, one had to configure the switch using the command "system mtu jumbo ####". Also, this was a command that had to be done manually on the switch and was not something you could write into the switch config. (In order to see the Jumbo MTU size, one had to type the command "show system mtu").

What I am trying to determine is the following:

1.) Has the configuration of "jumbo frames" changed with the 3850, 9300 switch series?

2.) Will inserting the line "System MTU ####" into a switch config, configure the switch for that MTU size in comparison to the old way of configuring "jumbo frames", when applied to a "new" switch? Or does one still have to configure the switch manually in order to apply the System MTU Size desired?

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Below are 3850 & 3560 switch output when issuing the command "show system mtu":

3850:

Global Ethernet MTU is 9000 bytes.

3560G:
System MTU size is 1500 bytes
System Jumbo MTU size is 9000 bytes
System Alternate MTU size is 1500 bytes
Routing MTU size is 1500 bytes

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Thanks in advance.

-gdston

[Reza Sharifi]

I don't have a 9300 to test with. As for the 3850 series, the command "system mtu xxxx" will turn on jumbo from for the entire switch. You would need to reboot the switch for the MTU to take effect.

Is that what you are asking?

HTH

[igdston.cisco]

Reza, thanks for the reply.
So, I understand that portion, as you explained. The other portion of my question, is whether or not the the command "System MTU ####" listed in a config file, will apply it to the entire switch.

[Reza Sharifi]

Hi,

The other portion of my question, is whether or not the the command "System MTU ####" listed in a config file, will apply it to the entire switch.

Yes, it does.  On smaller platform switches i.e 3750, 3850, 3560, 3650, the command can only be used globally, which means it is for the entire switch.  On larger platforms i.e 4500, 6500, 6800, etc, you can enable it per port.

Does this make sense?

HTH 

[igdston.cisco]

@reza
I believe, if we are understanding each other correctly, one can create a "template" config file with the command "system mtu ####", upload it to a "new" switch, reload it, and the switch will then be configured with Jumbo Frames on the entire switch. Correct?

[Reza Sharifi]

Yes, that is correct.

HTH

[igdston.cisco]

Awesome! Thanks Reza.

One last thing...just to confirm and clarify: on 3850, 9300 switches, this applies to all switchport speed types, i.e., Fast and Gigabit, correct?

[Deepak Kumar]

Hi, 

Q: 1.) Has the configuration of "jumbo frames" changed with the 3850, 9300 switch series?

Ans: Command is "system jumbomtu 9216"

Q:2) Will inserting the line "System MTU ####" into a switch config, configure the switch for that MTU size in comparison to the old way of configuring "jumbo frames", when applied to a "new" switch? Or does one still have to configure the switch manually in order to apply the System MTU Size desired?

Ans: If you enter the system mtu bytes global configuration command, the command does not take effect on the switch. This command only affects the system MTU size on Fast Ethernet switch ports.

Regards,

Deepak Kumar

[igdston.cisco]

Deepak, thanks for the reply.
Yes, I have read and understood that it will only apply to Fa ports.
So, the question then, how does one configure "jumbo frames" for Gig ports? Such as was possible with previous model switches?
Also, "jumbo" is not optional in the swtich models 3850, 9300, when setting "mtu size".

[girlpower]

After putting system mtu 9000 command on 9300 you get

)#system mtu 9000
Global Ethernet MTU is set to 9000 bytes.
Note: this is the Ethernet payload size, not the total
Ethernet frame size, which includes the Ethernet
header/trailer and possibly other tags, such as ISL or
802.1q tags.

thats shureley means thats it.....

I dont have to reload it do I?

when putting #show system mtu
Global Ethernet MTU is 9000 bytes.

Do i need to do anything else to allow jumbo frames?

[pkumari4]

Hello Manayat,

A reload is not required to change the system MTU. For more details kindly go through the below link.

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9300/software/release/16-11/configuration_guide/int_hw/b_1611_int_and_hw_9300_cg/configuring_system_mtu.html

I also verified the behavior on a 9300 switch in our lab:

Switch#config t

Enter configuration commands, one per line.
End with CNTL/Z.

Switch(config)#system mtu ? <1500-9198> MTU size in bytes

Switch(config)#system mtu 9198

Global Ethernet MTU is set to 9198 bytes. Note: this is the Ethernet payload size, not the total Ethernet frame size, which includes the Ethernet header/trailer and possibly other tags, such as ISL or 802.1q tags.

Switch(config)#^Z

Switch#show system mtu

Global Ethernet MTU is 9198 bytes.

Regards,

Priyanka

[dducamps01]

hello

to be noted system MTU on old switch family 3750 require a reboot to take effect.
NOT any more reboot needed since C3850 and of course younger prodcut as C9300/C9500

and with system MTU affect is on ALL ethernet port of the switch 

giga/ Tengiga

MAQ-SW07(config)#system mtu ?
<1500-9198> MTU size in bytes
MAQ-SW07(config)#system mtu 9198
Global Ethernet MTU is set to 9198 bytes.
Note: this is the Ethernet payload size, not the total
Ethernet frame size, which includes the Ethernet
header/trailer and possibly other tags, such as ISL or
802.1q tags.
MAQ-SW07(config)#

to be noted MTU MAX is NOT 9216 as other Cisco product bit 9198

MAQ-SW07#sh int mtu
Port Name MTU
Gi1/0/8 9198
Gi1/0/9 9198
Te1/1/3 PROBE10G-GR02-2 9198
Te1/1/4 TO MAQ-B22-Ten17 9198
Po5 9198
Po114 9198
MAQ-SW07#

[whistleblower14]

Hi,

this might be a stupid question, but - would there be a disadvantage when configuring fundamentally the system mtu on the switches for example to 9000 bytes and prepare so the device for any further purposes? because which MTU is used is based on the end devices (PCs, Servers), right? the switches are "only" in the transport way - of course when jumbo frames would be needed all other devices in the path have to support the higher values too!

[Joseph W. Doherty]

Yours, IMO, is not a stupid question.

The answer to your question is, "it depends" on what you consider advantages and disadvantages; including enabling jumbo support even if there's no jumbo traffic, at all.

At this point I could start listing potential effects when enabling jumbo support, to the switch itself, but as your question enters the subject realm why some switches, or switch line cards, are "better" than others, even when they have the same number of same bandwidth Ethernet ports, I would like to see what you think.

[whistleblower14]

hi @Joseph W. Doherty I've read your answer a few times now, but maybe it's because I'm not a native speaker - I need to apologize, but I don't understand this part?

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...but as your question enters the subject realm why some switches, or switch line cards, are "better" than others, even when they have the same number of same bandwidth Ethernet ports, I would like to see what you think.