MTU Issues on 9300 switch stack

[BillClark22]

Have a stack of 9300s running XE v17.12.03.  The system-wide MTU setting is 1500.  We have 4 VMware ESXi hosts connected and trying to set jumbo frames for the vMotion links for the hosts.  Several interfaces are set with "mtu 9198", while others are at the default of 1500.  When I configure one of these "1500" interfaces and use the "mtu 900" command, it says "invalid mtu specified".  I tried a wide range to see what it would accept, and for some reason the max MTU for these interfaces is 4580.  I have no idea where 4580 is coming from and have no idea why I can't set these interfaces to 9000.  Any ideas?

[marce1000]

      - FYI : https://bst.cloudapps.cisco.com/bugsearch/bug/CSCvg69090
                        This PPP stuff might be off topic , but I would compare the issue against the
                        latest advisory release for the switch anyway,

  M.

[David Ruess]

Hello,

This might help you:

Interface and Hardware Components Configuration Guide, Cisco IOS XE Dublin 17.11.x (Catalyst 9300 Switches) - Configuring System MTU [Cisco Catalyst 9300 Series Switches] - Cisco

The IP MTU seems to be maxed out at 1500 but the system MTU can go to 9000. Are the ports you are trying to configure routed (IP) ports? That could be why the limitation.

-David

[BillClark22]

No, they are not routed ports.  

[MHM Cisco World]

as I say before here in community cisco make mtu more complex 
anyway

system jumbo xxxx <<- this value is max mtu can config under interface so to make it 9000 you need to change this value then config mtu under interface 

MHM

[BillClark22]

I had thought that too, but I have 6 other interface ports that are used for vMotion that currently show an MTU size of 9198.  No warnings or anything in vSphere or errors on the switch stack itself.  I'm confused as to why 6 out of the 12 are set to 9198, but these other 6 I just can't set.  These other 6 interfaces are not part of a port-channel, are not routed ports and only belong to a single vlan.

[MHM Cisco World]

You need to set value to exact value 

I.e. use 9198 not use any other values.

MHM

[BillClark22]

Attempting to set the mtu value to 9198 for this individual interface gives me the same "% Invalid MTU specified".

[MHM Cisco World]

9500H#show system mtu

9500H#show interfaces mtu

Can I see output of both command 

MHM

[BillClark22]

I cut out the bulk of the other ports that aren't involved in this issue.  The interface I've been trying to modify is Gi3/0/42 (the only one set to 4580).  vMotion currently works fine between all the hosts.  In addition, no vMotion ports are part of a port-channel/etherchannel/trunk setup.  They are all individual interfaces that belong to vlan3, which is for vMotion connections only.

9300#sho system mtu
Global Ethernet MTU is 1500 bytes.

9300#sho interfaces mtu

Port Name MTU

Gi2/0/38 1500
Gi2/0/39 9198
Gi2/0/40 1500
Gi2/0/41 9198
Gi2/0/42 9198 (Host2-NIC3)
Gi2/0/43 1500
Gi2/0/44 1500
Gi2/0/45 1500
Gi2/0/46 1500

Gi3/0/39 9198
Gi3/0/40 9198 (Host5-NIC3)
Gi3/0/41 1500 (Host3-NIC3)
Gi3/0/42 4580 (Host4-NIC3)
Gi3/0/43 1500

Gi4/0/38 1500
Gi4/0/39 9198 (Host5-NIC6)
Gi4/0/40 9198 (Host2-NIC6)
Gi4/0/41 1500 (Host3-NIC6)
Gi4/0/42 1500 (Host4-NIC6)

Gi5/0/37 1500
Gi5/0/38 1500
Gi5/0/39 9198 (Host5-NIC7)
Gi5/0/40 9198 (Host2-NIC7)
Gi5/0/41 1500 (Host3-NIC7)
Gi5/0/42 1500 (Host4-NIC7)

[Jens Albrecht]

Hi @BillClark22,

is there any specific reason to leave the system mtu at 1500? If not, you could try to do it just the other way around.
After setting "system mtu 9198" all interfaces will use this as their default and you could then reduce it on certain interfaces if required.

HTH!

[BillClark22]

Yeah, I'm pondering that now with this being such a PITA.  I need to verify a few connections to firewalls and other devices before I pull that trigger though.  But I am leaning towards this more and more as I keep going down the rabbit-hole trying to find a fix.