In most cases to there is no need at all to reconfigure the system MTU size of a layer2 or layer3 PDU. The size of 1500 byte is the default. If configuring a MTU one usually does it on physical interfaces or logical interfaces. Like Ethernet or Interface VLAN if dealing with a cisco component. But a Nexus 5000 , and a Nexus 7000 series switch running NX-OS operating system will use some Jedi tricks if you do not pay attention:

Here the proof run on a n5k component:

n5k-switch# sh int e1/1 | i MTU
  MTU 1500 bytes, BW 10000000 Kbit, DLY 10 usec
n5k-switch# sh int vlan1 | i MTU
  MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec

It says clearly "My MTU is on physical and logical interfaces always 1500 bytes"

The component documentation suggests following:

!--- You can enable the Jumbo MTU 
!--- for the whole switch by setting the MTU 
!--- to its maximum size (9216 bytes) in 
!--- the policy map for the default 
!--- Ethernet system class (class-default).

n5k-switch#configure terminal
n5k-switch(config)#policy-map type network-qos jumbo
n5k-switch(config-pmap-nq)#class type network-qos class-default
n5k-switch(config-pmap-c-nq)#mtu 9216
n5k-switch(config-pmap-c-nq)#exit
n5k-switch(config-pmap-nq)#exit
n5k-switch(config)#system qos
n5k-switch(config-sys-qos)#service-policy type network-qos jumbo

Below the method described on how to verify a MTU size a nexus switch is using really:

n5k-switch# sh queuing int e1/1 | i MTU
    q-size: 469760, HW MTU: 9216 (9216 configured)
n5k-switch# sh int e1/1 | i MTU
  MTU 1500 bytes, BW 10000000 Kbit, DLY 10 usec