upgrading Multi server Zimbra network addition

[ECB]

HI To all

We need Advice on upgrading our Zimbra Servers.

Our Servers are setup as follows:

Branch A

Server running Ubuntu 8.0.4 with Zimbra 6.0.4 network addition as the LDAP master with 800 Mailboxes on DomainA.com


Branch B

Server running Ubnutu 8.0.4 with Zimbra 6.0.4 , network addition this server is a LDAP replica Form BranchA and has 600 mailboxes on DomainB.com.


Both servers are setup with additional scripts on postfix to block a few mailboxes from sending or receiving from the Internet.

What is the best practice for upgrading? What files should we backup before upgrade for DR.

What version should we upgrade too?

What server should we upgrade first?

Thanks

Meron