Why block TFTP at your borders? To keep people from loading new versions of IOS on your routers? ;)
Not trying to be flippant, but what's the basis for this?
This is a really good question :) In our particular case, it was not to protect the network as others suggested. We do ACL our equipment, keep updated code, use private IPs were necessary, etc. We're a University network, but we're not completely insane ;) Of course we don't let random hosts TFTP to our gear... A while ago (18 months maybe?) our security team argued that filtering TFTP connections between subnets on our campus would slow down the spread of computer worms/viruses as many were using TFTP as part of their propogation vector. The decision was made that the trade off between the end-to-end principle (we didn't have a good counter at the time citing a particular application that was used and would break) and helping contain virus outbreaks was worth filtering, so the filter was put into place. No one has complained yet, so the filter has stayed in place. Eric :)