Hello,
Here are the Symantec Knowledgebase Articles for the same:
1) How to configure a Symantec Endpoint Protection Manager (SEPM) 12.1 client to use a proxy for LiveUpdate
http://www.symantec.com/docs/TECH167247
2) How to setup the Symantec Endpoint Protection Manager to use specific proxy settings for LiveUpdate
http://www.symantec.com/docs/TECH102335
3) Configuring Symantec Endpoint Protection Manager to connect to a proxy server to access the Internet
http://www.symantec.com/docs/HOWTO55201
4) Specifying a proxy server for client submissions and other external communications
http://www.symantec.com/docs/HOWTO55363
5) What is the ISAPI proxy setting?
http://www.symantec.com/docs/TECH163729
6) How to bypass the IIS proxy after upgrading to Symantec Endpoint Protection Manager 12.1
http://www.symantec.com/docs/TECH162324
7) Required exclusions for proxy servers to allow Symantec Endpoint Protection to connect to Symantec reputation and licensing servers
http://www.symantec.com/docs/TECH162286
Hope that helps!!