When updating the firmware and bootcode of your CISCO switches or routers, TFTP is very useful. Sometimes, even the only option available. But, many have my same problems of upgrade failed with error "Tftp server unreachable." This is how I've just fixed my problem with TFTP server this morning...
I'm using Solarwinds TFTP Server, you may download it for free from the link below:
The installation is straight forward, and the default settings are good to run. (so, I won't explain much here). The main point is your Windows firewall needs to be configured correctly to make it works, or it will block your FTP access.
1) If you have a 3rd party firewall, temporarily disabled it. (Or, if you know how to make a firewall exception to allow the TFTP service app to communicate. If so, follow the principles as below)
2) I assume your only Firewall is Windows Defender. On Windows search box, search for "Firewall" and click on "Allow an App for Windows Firewall".
3) Press Change Settings, then press the Allow another app... button
4) Press Browse, and select the SolarWinds TFTP Server service, named "SolarWinds TFTP Server.exe"
5) Go back to Network Types button and select the network environment that you wants to have the firewall rule applied. It must match your current network. If not sure, you may click Private and Public.
6) Click Add and then OK.
7) Start your Solarwinds TFTP server and start the CISCO firmware and bootcode upgrades.
The principle is to configure your firewall to allow communication of your TFTP server. However, the default settings e.g. for Solarwinds, is just allow access to the Configuration interface app but no firewall rules being created for the actual TFTP server service.