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Exchange HUB & CAS redundancy

padas6 asked
I have One Hub Transport Server & One One CAS server . I want to make it both redundancy, means, both the Server should be Hub & CAS roles along with CAS Server make its NLB .

Please suggest how to proceed for make the setup successful.
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Mahmoud SabrySenior IT Systems Engineer

install the hub role on the cas server and the CAS role in the hub server

HUB will be load balanced by default
CAS: you must install NLB on both servers, and then add both servers to the NLB cluster

As per msabry06 but you will need to create a CASARRAY with is an active directory object that reflects the load balanced CAS servers.


This is a walkthrough on how to do it.
Mahmoud SabrySenior IT Systems Engineer

@Radweld: this is for exchange 2010 not exchange 2007



Hi msabry06,

Thanks for your reply & yes this is Exchange 2007, just clarify some points,

1. I have to install CAS or HUB first?
2. After installed the Exchange Roles can I installed NLB in both the Servers?
3. If yes, I have installed NLB first or Roles ?
Senior IT Systems Engineer
1 : doesn't matter which role installed first

2: yes you can install NLB after exchange roles installed

3: doesn't matter you install NLB or roles first, but you should prepare your topology and deep understand NLB and your exchange environment, find the below article for how to use NLB with exhange 2007 server roles





Thanks msabry06
Maen Abu-TabanjehNetwork Administrator, Network Consultant
Top Expert 2011

for CAS there is basically a choice between WNLB and and HLB. Hub load-balances by default outbound mail. For inbound Internet mail, one solution is to use MX records with equal value.

Here's some some of the newer in-depth information on LB’ing CAS servers in Exchange 2010 documentation on TechNet

Understanding Load Balancing in Exchange 2010

Load Balancing Requirements of Exchange Protocols

Uncovering the new RPC Client Access Service in Exchange 2010 (Part 1)

Load Balancing Exchange 2010 Client Access Servers using an Hardware Load Balancer Solution (Part 1)


If its exchange 2007 then just create the NLB and leave it at that. It doesn't matter how you install the roles, just make sure your using the same version of media.


Hi, Yesterday night, i did the Hub Installation in CAS box, when I tested my new Hub Transport Server functionality,  I am not able to send the mail to external domain , but receving from outside, . I am not able to telnet my smart host on port 25, but before doing this activity, everything has been  congured in firewall &  my iron port by the help of security team.

Please suggest what could be possible cause of this issue.

One more thing , apart from the default & client receive connector, other custom relay connector is not replicated to other hub transport Server. Shoud I need to configure those connector manually.

Note : NLB still not configure, planning to do next week during the installtion of CAS Server.
Mahmoud SabrySenior IT Systems Engineer

receive connectors are per server basis, so you must create the receive connectors for the new server so that it can accept connections like the first server

for the new HUB server, you must add it in the source servers tab in the send connector so that it's able to send emails to the internet

make these changes , restart microsoft exchange active directory topology service on the new server and let us know what is up


Default & client receive connector already created by default after installing the Hub Transport Server in a CAS box, only configuration has been made same as another hub TRansport Server.

Also added the source server list in send connector , forget to restart microsoft exchange active directory topology service , lets try & let you know the update.


No Result, telnet smarthost 25 is saying connection is lost, but it is pinging.
Mahmoud SabrySenior IT Systems Engineer

can you telnet the the old server from the new server on port 25, if yes, then the problem is in the firewall not in the server, let me know

Connection lost means your smart host is rejecting connection. This means there is a configuration error or the wrong end points been specified (ip of the new hub) or of your load balancing them, the VIP of the array.