MTU on SVI of C9500-48Y4C

[tweep9141983]

Hello Everyone,

I found the below previous discussion about MTU on SVI of 6500 switches.

https://community.cisco.com/t5/switching/mtu-size-on-vlans/td-p/937492?fbclid=IwAR19wmQaVxEo6lMCtmTqbA0L-fJaAgU3Cdud8IYl2y-IMnCUiWyoBQJFoMo

Now I also have the dilemma with our 9500 switches wherein we needed MTU 9000 on physical interfaces and VLAN interfaces.

The Physical interface is fine and already configured with MTU 9000 however no way to configure in VLAN interfaces.  See below:

9500#sh run int tw2/0/47
Building configuration...

Current configuration : 221 bytes
!
interface TwentyFiveGigE2/0/47
description ECX Fabric 1 (TY9 to HK5) 21635303-A PRIMARY
switchport trunk allowed vlan 100,101,200,201,777
switchport mode trunk
mtu 9000
spanning-tree vlan 777 port-priority 32
end

9500#sh run int vlan 100
Building configuration...

Current configuration : 65 bytes
!
interface Vlan100
ip address 172.30.10.101 255.255.255.0
end

9500#

9500#conf t
Enter configuration commands, one per line. End with CNTL/Z.
9500(config)#int vlan 100
9500(config-if)#mtu 9000
^
% Invalid input detected at '^' marker.

9500(config-if)#mtu ?
% Unrecognized command
9500(config-if)#ip mtu 9000
^
% Invalid input detected at '^' marker.

9500(config-if)#ip mtu ?
<256-1500> MTU (bytes)

9500(config-if)#ip mtu ^Z
% Incomplete command.

9500#

Please let me know if more information is needed.  Thanks in advance.

[Georg Pauwen]

Hello,

I think you can only do 'system mtu' which sets the MTU for all interfaces. SVIs do not allow you to set the MTU...

[balaji.bandi]

Restrictions for System MTU

When configuring the system MTU values, follow these guidelines:

• 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.

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9500/software/release/16-8/configuration_guide/int_and_hw/b_168_int_and_hw_9500_cg/b_168_int_and_hw_9500_cg_chapter_011.html

[tweep9141983]

Thanks for the update.. I am able to configure the MTU on the physical interface.. and currently confirmed fine with our VM environment using Jumbo Frames.. only issue now is the SVI/VLAN interfaces wherein we need to configure L3 routing to it and at the moment, it won't allow MTU size > 1500 even though I already configured MTU 9000 to the physical interface associated to the VLAN.

9500#sh run int tw2/0/47
Building configuration...

Current configuration : 221 bytes
!
interface TwentyFiveGigE2/0/47
description ECX Fabric 1 (TY9 to HK5) 21635303-A PRIMARY
switchport trunk allowed vlan 100,101,200,201,777 <<<<< VLAN 100 is where I need MTU 1500
switchport mode trunk
mtu 9000 <<<<< the Switch allowed me to configure MTU to the physical port
spanning-tree vlan 777 port-priority 32
end

9500#

[andrew.butterworth]

You need to set the global system MTU and then set the IP MTU on your SVI (and physical L3 interfaces - no switchport).

I think by default the IP & IPv6 MTU follows the system MTU - i.e. if you set the system MTU to be 9100, each L3 interface is also 9100.  When I configured the system MTU on a C3650 I lost all the OSPF & OSPFv3 adjacencies as all the neighbours were 1500 default.  Changing each L3 interface so they matched solved the issue.

The C3560X/C3750X series lets you set the global system MTU & the global routing MTU separately,