System MTU vs Interface MTU

[Rady]

Hello folks,

We are checking max MTU we can achieve .. As you can see, we can configure the MTU under interface to be 9000 while only 1552 through system MTU.


What's the different between both?, Is there anyway to upgrade "system MTU" to accept jumbo frames?. as we need to allow 9000 to be able to ping with don't fragment bit size 9000.

LAB_SW#show version
Cisco IOS Software, Catalyst 4500 L3 Switch Software (cat4500-IPBASE-M), Version 12.2(25)SG4, RELEASE SOFTWARE (fc1)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2008 by Cisco Systems, Inc.


LAB_SW (config)#system mtu ?
<1500-1552> MTU size in bytes


LAB_SW#conf t
LAB_SW(config)#inter Gi1/7
LAB_SW(config-if)#mtu ?
<1500-9198> MTU size in bytes

[MHM Cisco World]

https://www.cisco.com/c/en/us/support/docs/switches/catalyst-6000-series-switches/24048-148.html

it depend on platform, some support system and other support under interface

[Rady]

Thanks dear.

What would be the case if I configure MTU of 9000 under the interface while system MTU max size is 1552?

[MHM Cisco World]

always the interface MTU override the system MTU

[balaji.bandi]

The Catalyst 4500 series switch allows you to configure a maximum of 32 different maximum transmission unit (MTU) sizes systemwide. This means that the maximum number of different MTU sizes that you can configure with the system mtu, mtu, ip mtu, and ipv6 mtu command on all Layer 2 and Layer 3 interfaces combined is 32.

suggest to look config guide :

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4500/12-2/53SG/configuration/config/sw_int.html

[Joseph W. Doherty]

As others have posted, interface jumbo Ethernet MTU, if supported on that platform, should be the MTU for that interface.

System MTU, usage, I believe, might vary.

On low-end Cisco switches, I've seen it used to allow jumbo Ethernet MTU on all interfaces.  (I.e. cannot individually change specific interface MTU).

System MTU, in conjunction with interface MTU, might (I'm unsure) inform the device what MTU it can use for device generated traffic.  (BTW, there's usually not much in the way of device generated traffic that would really benefit from jumbo Ethernet, excluding, perhaps, passing [the initial] Internet route tables to a BGP peer.)