Problem About IP MTU

[rogetnoprl]

hello, I am confused with mtu settings,here are problems:

a windows server is in vlan 21,and the interface ethernet to the server is 1500

however, when i change int vlan 21 ip mtu to 1280,and I run "ping IP -f -l 1472" can still get reply from the other host.the other host is in another VLAN routed by this switch(cisco 4506),

it seems there is nothing happend when I change the IP mtu of this VLAN interface, so I want to know what happened to the IP packet which was translated  when the switch has vlan interface IP MTU 1280? Did it fragmented？ I believe that "ping -f" set the DF bit, it should not to be fragmented

[flokki123]

hi roget,

i guess the receiving interface on the switch is still set to mtu 1500, right?

what if you set this to 1280 as well? does the ping still work?

and yes, -f with the ping command tells the device to not fragment the packet.

florian

[rogetnoprl]

Yes.The receiving interface MTU is 1500. and I Can't set interface MTU to 1280, 

sw-a5-01-C4506-E(config-if)#mtu 9198
% Interface GigabitEthernet4/38 does not support user settable mtu.
sw-a5-01-C4506-E(config-if)#mtu 1280
                                                ^

% Invalid input detected at '^' marker

[rogetnoprl]

there is someting i found,

if I set vlan IP mtu to 1280 ,and the vlan has an IP, and i Ping VLANIP -f -l 1500 got "request time out" reply.ping VLANIP -f -l 1208 got reply

so what happend to the packet(to other server) if the vlan ip mtu is 1280? VLAN ip is default gateway of this VLAN