Impact of changing core MTU

[BurbManDan]

Greetings,

A few questions regarding changing MTU on core switch for 802.1Q tunneling support... System MTU is currently default (1500), switch in production network :

1. Will changing system MTU to 1508 result in loss of inband management connection?  (since I connect to this switch via SSH in another state

2. Is this change disruptive to other connectivity?  (In other words, will this "take down the network" - how long?)

3. Does this require a reset, like a reload? 

[Reza Sharifi]

Hi,

What is the switch platform you are using?

On smaller switches like 3800, 3700, 9300, changing the MTU requires a reboot as it applies to the whole switch.

HTH

[BurbManDan]

This is a cisco WS-C3750X-48 stack running ipbase
1 54 WS-C3750X-48 15.0(2)SE8 C3750E-UNIVERSALK9-M

[Reza Sharifi · Accepted solution]

So, you need a reboot after deploying the config. Changing MTU should not affect the inband management connection but I recommend doing it during off-hours just in case.

HTH

[BurbManDan]

Thanks Reza!  

Just to be sure I understand, it sounds like when I run:

#system mtu 1508

nothing will change until after I follow that up with:

#reload

Correct?

[Reza Sharifi]

nothing will change until after I follow that up with:

#reload

Correct. 

The system MTU setting is saved in the switch environmental variable in NVRAM and becomes effective when the switch reloads. Unlike the system MTU routing configuration, the MTU settings you enter with the system mtu and system mtu jumbo commands are not saved in the switch Cisco IOS configuration file, even if you enter the copy running-config startup-config privileged EXEC command. Therefore, if you use TFTP to configure a new switch by using a backup configuration file and want the system MTU to be other than the default, you must explicitly configure the system mtu and system mtu jumbo settings on the new switch and then reload the switch.

Also, check your license level as the command is not supported on switches with LAN base feature set.

Link:

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750x_3560x/software/release/12-2_55_se/configuration/guide/3750xscg/swint.html#49711

Reza