Question : Exchange 2007 HT / CA server high availability issue

I've facing an interesting problem I can't pin down.

Here's some information on the environment:

Firewall:  FortiGate 500a

Exchange 2007 SP1 - 4 Servers
2 - node Mailbox SCC
2 - node HT/CA


Ok, so the problem I'm running into is with load balancing for the 2 HT/CA servers.  I'm doing this at the firewall via a VirtualIP mapping.  The external IP is mapped to two internal IPs in a load balancing configuration.

We'll call the Exchange03 (192.168.1.3) and Exchange04 (192.168.1.4).

Exchange03 is a physical server.  Exchange04 is a virtual server running on ESX.

When both IPs are entered in the VirtualIP mapping ( X.X.X.X -> 192.168.1.3 - 192.168.1.4 | load balancing enabled) and both servers are online mail flows wonderfully.  When both IPs are entered and Exchange04 is down, mail stops.  Same setup, Exchange03 is down mail flows with no issue.

When only Exchange03 or Exchange 04 is entered into the Virtual IP mapping configuration, mail flows normally so long as that server is online.

For some reason, it looks like when I try to load balance at the firewall level and Exchange04 is down, mail flow stops (incoming only | outbound works perfectly).

I guess what I'm trying to track down is what would prevent Exchange03 from receiving mail when Exchange04 is down?  Would the firewall not pass the email down to the node that is up?

TIA

Answer : Exchange 2007 HT / CA server high availability issue

You need to load balance your CAS servers using Windows NLB and then have a nat rule from your public ip to the nat ip address. This way, windows NLB balances the load between the servers.

Rajith.