Cisco Cat 9300 does not permit to modify mtu value on vlan interface

[SYM Networking Engineer]

Hi everybody, i am facing an issue, in the company we have Catalyst switches 9200 and 9300, we have to put specific mtu value on physical and vlan interfaces, but only the Catalyst 9200 permit to do it, is there a additional configuration to do it on the Catalyst 9300?

Catalyst 9200

Model: C9200-24T

Firmware version: 17.06.03 CAT9K_LITE_IOSXE

Catalyst 9300

Model: C9300-24T

Firmware version: 17.06.04 CAT9K_IOSXE

Regards

[Flavio Miranda]

@SYM Networking Engineer 

 Why Cisco do not allow per vlan mtu change on 9300 I failed to find a doc. It seems you can set MTU with "system mtu" command or per interfaces.

Did you try

int vlan 434

 ip mtu  xxx

?

Device# configure terminal
Device(config)# interface gigabitethernet 0/1
Device(config-if)# ip mtu 900 

evice# configure terminal
Device(config)# system mtu 1600
Device(config)# exit

[Flavio Miranda]

@SYM Networking Engineer 

 I have found this doc where we can see this table. It seems you can setup MTU for L3 SVI but L3 SVI, as you are trying, is not listed here.

https://www.cisco.com/c/en/us/support/docs/switches/catalyst-9500-series-switches/217233-troubleshoot-mtu-on-catalyst-9000-series.html

Background Information

MTU Summary Table

Total Fram Size = MTU + L2 Header

Port Type	Default MTU - Bytes	Configured MTU - Bytes	L2 Header	Total Frame Size
L2 Access	1500		18	1518
		9216	18	9234
L2 Trunk	1500		22	1522
		9216	22	9238
L3 Physical Port	1500		18	1518
		9216	18	9234
L3 SVI	1500		18	1518
		9216	18	9234
IP MTU on L3 port	1500	Range is supported.	18	Based on the ip mtu configured value.

[MHM Cisco World]

You can not, 

it default 1500.

MHM 

[DanielP211]

Hello!

The correct command is
int vlan XXX

ip mtu 9100

BR

[balaji.bandi]

First, what is the use case for SVI to require MTU changes?

[M02@rt37]

Hello @SYM Networking Engineer 

The ip mtu command is available under interface configuration mode on the C9300, you can use it to set the MTU for IP packets on specific interfaces, including VLAN interfaces.

On the other hand, the command system mtu jumbo <value> is used to configure the global Layer 2 MTU (jumbo MTU) on a switch, specifically for interfaces handling jumbo frames. This setting defines the maximum size of Ethernet frames  that can be transmitted on the switch's interfaces. !!! Need a reload !!!

For routed interfaces, you may need to configure ip mtu separately if IP traffic needs a specific MTU.

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9300/software/release/17-6/configuration_guide/int_hw/b_176_int_and_hw_9300_cg/configuring_system_mtu.html

The difference in MTU configuration between the C9200 and 9300 switches stems from their architectural design and intended use cases. C9200 supports the mtu command on individual L2 interfaces, providing more flexibility for per-interface MTU settings, which aligns with its role as a cost-effective, access-layer switch. In contrast, the C9300 is designed for more advanced features, such as SD-Access and large-scale deployments, and uses a global MTU management approach. On the 9300, the system mtu and system mtu jumbo commands configure L2 MTU globally, ensuring consistency across interfaces and simplifying management for features like VXLAN or MPLS. This difference reflects Cisco's design philosophy, where simpler devices like the 9200 allow for granular configuration, while higher-end switches like the 9300 prioritize scalability and uniformity. If granular MTU settings are required on the 9300, global MTU configurations should be adjusted, as the mtu command is not supported per interface.

[shambhu.kumar]

Even if you configure ip MTU under VLAN interface in 9300 , it will not go beyond 1500. What is exact requirement,where it needed to configure MTU under VLAN level?

9300 have

In 9200 you have both options,