1. My Environment:
1 dell desktop-Optiplex
1 dell laptop-Latitude

Networking proxy:

http://my-proxy.my-domain:99

Operation System:
F11-XFCE

2. Issues:
Seemed as a bug of yum/F11 or an issue of my networking, yum always hang in the office.
Also my laptop can normally update with yum at home(without proxy).

3. Solved:
The environment of my Desktop is cleaned with http_proxy set.
An error came out when I tried to re-run “yum update”:
Error: Cannot retrieve repository metadata (repomd.xml) for repository: fedora. Please verify its path and try again
Google helps,
How to fix the “Cannot retrieve repository metadata (repomd.xml)” error in Fedora?
Unexpected,yum can update and all goes well. I redo it on my laptop and also get the point.

4.WorkAround:

• You need to edit two of your repository files: /etc/yum.repos.d/fedora.repo and /etc/yum.repos.d/fedora-updates.repo. Now un-comment all the lines that start with  the term baseurl and place a comment before all lines that start with mirrorlist. This should be done for both the above files.

• Now edit your /etc/hosts file and append the following to it’s contents:
  

  80.239.156.215          mirrors.fedoraproject.org
  213.129.242.84          mirrors.rpmfusion.org

5.Conclusion:
Issue of yum, or the solution of mirrorlist can’t go well with proxyed-envrionment.