Baby Giants and Jumbo Frames

Someone asked me about Jumbo frames so I thought I would take a bit of a look (partly because I have to look at Baby Giants anyway). Here is what I have found so far:

• Baby Giants have an MTU of up to 1600 bytes. Jumbo frames can be over 9000 bytes (the exact size depends on the switch platform and possibly IOS).
• Some Cisco switches and routers don't support baby giants or jumbo frames, usually because of ASIC limitations.
• If you just want baby giants so that you can put a single MPLS tag on a frame you can try changing the interface to use 802.1q trunking and then place your MPLS traffic in the native VLAN. This changes the MTU to 1504 (so it fits one tag) and is supported on most switches.
• The interface counters will count the baby giant or jumbo frame as being over-sized even if the interface can support it - this is cosmetic.
• Some devices allow you to set a separate MTU for 10/100 and Gigabit interfaces. One of these is the 3750. For the 10/100 interfaces you use the system mtu number command. For the Gig interfaces you use the system mtu jumbo number command. If you set the system mtu but not the system mtu jumbo then the Gig interfaces use the system mtu. The reason for the two commands is that the 10/100 interfaces only support a lower MTU than the Gig interfaces. Other than trying the commands or looking up on the Web there does not seem to be a way to find out what the maximum acceptable sizes are.