Jumbo frames between 2960 and 3850

[Pierre Dlbl]

Hello everyone,

We would like to set up jumbo frames between a cisco 2960 and a cisco 3850.

On the 2960 :

2960#show system mtu

System MTU size is 1500 bytes
System Jumbo MTU size is 9198 bytes
System Alternate MTU size is 1500 bytes
Routing MTU size is 1500 bytes

On the 3850 :

3850#show system mtu

Global Ethernet MTU is 9198 bytes.

A ping from the server plugged in to the 3850 switch to the ISCSI card of the storage server is not working.

Apparently it works up to 8972 but no more.

Could you help me ? Do I miss something in the config ?

Thank you very much.

[Rick1776]

What is the MTU on the server NIC set at?

[Rick1776]

Also how are you testing the MTU?

[Pierre Dlbl]

Hi Rick,

Thanks a lot for your answer. I think it's the way we tested it that was wrong. We tested from the server by pinging this way :

ping x.x.x.x -l 9000

ping x.x.x.x -f -l 9000 was giving :

Packet needs to be fragmented but DF set

ping x.x.x.x -l 8972 is working.

From what i've read here : 

https://social.technet.microsoft.com/Forums/windowsserver/en-US/12c1a2da-762e-45fb-9fb5-bc940f940ca5/jumbo-frames-and-networking-recommendations-for-hyperv-w-iscsi?forum=winserverhyperv

The good way to test jumbo frames from Windows is with -l 8972. Is that right ?

Thank you!

[Rick1776 · Accepted solution]

That is the correct way.

I would also look at using iperf windows version.https://iperf.fr/ With this tood you can also specify the MTU and gives me more reliable results.

Remember that if your MTU is set at 1500 anywhere on the LAN it will drop packets unless each switch in line have jumbo frames enabled.

[Pierre Dlbl]

Thanks a lot Rick!