Exchange 2000 and GC server down

Here is my configuration:

My network is a small LAN (150 user and 3 servers), only 1 domain.

Server A: DC Roles(Domain Name Master, Escheme Master)+ GC
Server B: DC Roles(other 3 roles of the forest) + GC
Server C: Exchange 2000 Member server

Questions ?
1-Server A FAIL:
If Global Catalog server (Server A) go down the client computers can't
contact de Exchange server. So how can I force de Exchange server to use the
other Global Catalog server (Server B) ?

How my Exchange Client should be able to fail over to the GC B?

Note: I don't want to restart  server B or C

2- In Exchange 2000 Administrative console --- Server Property---- Directory
Access appear the topology of my Domain:

What is the function and difference of:?
 DC
 GC
 Configuration server
   
How can I mitigate the domino effect ? 

I want to add some redundancy for state available the Exchange server in
case of one GC server go DOWN. HOW do it?

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