Hello Azm
This depends on the platform. On the 3750 for example, the MTU cannot be changed on an individual interface, but is implemented globally. On the higher end catalyst switches (4500, 6800 etc) and always depending on the supervisor and IOS used, you can configure it on a per interface basis. In either case, you can verify the MTU on aper interface basis by using the show interfaces command. For example:
7609#show interfaces gigabitEthernet 1/1
GigabitEthernet1/1 is up, line protocol is up (connected)
Hardware is C6k 1000Mb 802.3, address is 0007.0d0e.640a (bia 0007.0d0e.640a)
MTU 9216 bytes, BW 1000000 Kbit, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255
Notice the MTU size is well above the normal 1500 bytes, so this port is configured to allow for jumbo frames.
Another useful command is the following:
Switch# show system mtu
System MTU size is 1546 bytes
System Jumbo MTU size is 9000 bytes
Here you can see that the jumbo MTU size has been configured to 9000 bytes. This essentially means that all ports have been configured to accommodate jumbo frames. More information on jumbo frames can be found here.
There are situations where you require the increase in MTU size. One situation where I have come across it is on a metropolitan area fibre optic network. This network interconnects multiple organisations and connects them to a common connection to the Internet. Because QinQ is being used (which uses additional bits in the frame and increases its size beyond 1500 bytes to add a second VLAN tag) in order to send multiple client VLANs over single MAN VLANs, the MTU is raised up to well above the 1500 mark to accommodate all sizes of frames as well as to provide for more efficient transmission and the elimination of any possible fragmentation.
Whether or not you have jumbo frames does not directly affect the DF bit. If jumbo frames are accommodated, then even if the DF bit is set, it is unlikely that any fragmentation will occur, because it is not necessary. Whether or not the DF bit is set depends strictly on the application being used. Situations where the DF bit is used is in Path MTU discovery as well as in situations where overhead must be avoided. Fragmentation generally adds CPU resource overhead as well as data overhead (in the form of additional headers needed for each fragment).
I hope this has been helpful!
Laz