We noticed one thing, while testing  S5720-HI switch with V200R007C00SPC500 version onboard.

We configured L3 interface from L2 int using undo portswitch command.
And applied mtu command.



[S5720-HI-ACC1-GigabitEthernet0/0/36]di th
#
interface GigabitEthernet0/0/36
undo portswitch
mtu 3000
ip address 10.10.100.1 255.255.255.0
ospf enable 1 area 0.0.0.0


But command display interface shows us that:



<S5720-HI-ACC1>dis int g 0/0/36
GigabitEthernet0/0/36 current state : UP
Line protocol current state : UP
Last line protocol up time : 2015-07-14 09:34
Description:
Route Port,The Maximum Frame Length is 9216



The Maximum Frame Length is 9216.   9216 is constant and independent of mtu setting parameter.

Maximum Frame Length  is not MTU, which is set by mtu command.

Real mtu (ip)  can be checked by protocol commands.



Display ospf brief

Interface: 10.10.100.1 (GigabitEthernet0/0/36)
Cost: 1       State: DR        Type: Broadcast    MTU: 3000 
Priority: 1
Designated Router: 10.10.100.1
Backup Designated Router: 0.0.0.0
Timers: Hello 10 , Dead 40 , Poll  120 , Retransmit 5 , Transmit Delay 1





But if we use real L3 interface,  on AR router for example,  or  vlanif interface:





interface GigabitEthernet0/0/2
mtu 9202
ip address 10.10.100.2 255.255.255.0
ospf enable 1 area 0.0.0.0
#
return




[AR3260-GigabitEthernet0/0/2]dis int g 0/0/2
GigabitEthernet0/0/2 current state : UP
Line protocol current state : UP
Last line protocol up time : 2015-07-14 09:34
Description:HUAWEI, AR Series, GigabitEthernet0/0/2 Interface
Route Port,The Maximum Transmit Unit is 9202


We set mtu 9202 and this is shown by display interface command.