When I test with Cisco 2960 series switches or with ISR routers, then if port receives an Ethernet frame larger than its MTU value seen in sh int output, then device drops this frame and increases the giants counter. In my opinion this is an expected behavior. However, for example some Linux servers(different NIC vendors and thus different kernel modules) still process frames with larger MTU than seen in ip link show or ifconfig output and send fragmented reply back.
What is a correct behavior? What should a device do when it receives an Ethernet frame larger than its MTU? I would expect it to either silently drop this frame or behave like Cisco equipment which nicely counts giant frames.
Best Answer
The ethernet standard (IEEE 802.3) calls the large frames damaged because the bits where the FCS is supposed to be don't match the CRC of the bits coming before them. What you see the Cisco switch do is correct for the default.
Some vendors have adopted Jumbo Frames, and what you see on the servers is the use of those. Unfortunately, there is no actual standard for Jumbo Frames so the limit on the size can vary from manufacturer to manufacturer, and even on different models from the same manufacturer. For example, see Jumbo/Giant Frame Support on Catalyst Switches Configuration Example. That document tells you that the Cisco 2960 series switch can be configured to use an MTU up to 1998 bytes on 10/100 Mbps interfaces, and up to 9000 bytes on 1000 Mbps interfaces.