Shockwave, the MTU indicates the Maximum Transmission Unit, or the biggest packet that can be sent across an interface. You can shrink the MTU down so far that the simple overhead of the IP packet and Ethernet frame are so big that it really can't carry much (if any) data. Screws everything up.
If data is too big to get sent in one packet it is fragmented across several. Some things aren't fragmentable, like a few types of routing protocol packets. They just don't get transmitted, basically causing your network to shut down.
- G