Jumbo Frames on Cisco 9300

[TRACY HARTMANN]

We have a need for Jumbo frames on a Cisco 9300.   The issue is we could turn it on by interface on our 4500 switch but now it looks like a global command of system mtu on the 9300.   I am concerned what kind of issues I will have if I turn on this global command.  There is no individual command per interface?

[Georg Pauwen]

Hello,

it looks like you can override that value on routed interfaces only. Either way, it shouldn't matter, as the system MTU value only configures the maximum value supported, so it has no effect on frames that are smaller, such as the default 1500 frames...

Configuring System MTU

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9300/software/release/16-6/configuration_guide/int_and_hw/b_166_int_and_hw_9300_cg/b_166_int_and_hw_9300_cg_chapter_0100.html

[TRACY HARTMANN]

Ok so your saying I can use the system mtu and it won't affect any of the interfaces on the switch because the ports that can't use Jumbo frames won't ?

I am concerned when we go from a statement that was per interface and now goes to a global command.

Thanks

[TRACY HARTMANN]

I added system mtu 9100 and everything seemed fine for two days and then we started to get packet drops on one interface.  The interface we connected to a Riverbed which connects to  a Cisco Router that supports the sites Wan setup.  I tried to change the Riverbed MTU to 9100 but seemed to still have an issue.   Any suggestions on the Cisco router that needs to be updated?

Thanks

[Reza Sharifi]

Is this a Catalyst 9300? if that is the case the Jumbo frame command is a global command and covers all interfaces.

• The device does not support the MTU on a per-interface basis.

• If you enter the system mtu bytes command in global configuration mode, the command affects all the switched and routed ports on the switch.

• link:
• https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9300/software/release/16-6/configuration_guide/int_and_hw/b_166_int_and_hw_9300_cg/b_166_int_and_hw_9300_cg_chapter_0100.html
• HTH

[pkumari4]

Hello,

9300 switch does not support the MTU on a per-interface basis, Therefore, you need to change the MTU globally.The system MTU can be seen as a tunnel diameter, if you increase the MTU, the switch will be able to receive and transmit frames from end devices with MTU above the default value, and it still will be able to forward/receive normal MTU traffic. Therefore, it is not an expected impact in interfaces was are not receiving frames with higher MTU.

You also need to check at the Riverbed end, what is the configuration.

regards,

Priyanka

[justo]

Hi Tracy, i'm no expert but have seen my fair share of riverbeds. I have only ever used 1500 MTU on the riverbeds I deploy. Figured having more than that might cause additional headaches when i'm setting a tunnel interface to 1470 anyway (ipsec overheads) 

• Make sure the interface bandwidth matches
• If you have a the riverbed physically in-path between the router and switch, make sure you have a crossover cable, as per the steelhead deployment guide.
• MTU's match

Might be a question for riverbed support?

[TRACY HARTMANN]

There is no way to change the MTU per interface after I turn the global command on?   I was going to try this again and turn the Jumbo frames on but then change all of the interfaces to MTU 1500 except the two I need Jumbo on.

[justo]

Hi Tracy, On a 9300 you can only change the MTU system wide:

Device(config)# system mtu xxxx

 Or on a routed interface only.

Device(config-if)# ip mtu xxxx

 https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9300/software/release/16-6/configuration_guide/int_and_hw/b_166_int_and_hw_9300_cg/b_166_int_and_hw_9300_cg_chapter_0100.html