dual site DAG configuration

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B

binbash



New to exchange and looking at deploying an exchange organization containing 6 vmware virtual machines in two sites (active/active) connected via redundant 10Mbps links to serve ~350 users, as follows:

SITE A

1 x CAS / HUB / FSW

2 x Mailbox servers

SITE B

1 x CAS / HUB / FSW

2 x Mailbox servers

The 4 mailbox nodes would make up 2 x 2-node DAGs.

DAG 1

Would have 1 node at SITE A and the other at SITE B. SITE A would be active under normal operation serving local users, the CAS/HUB server at SITE A would serve as the FSW for DAG 1.

DAG 2

Would have 1 node at SITE A and the other at SITE B. SITE B would be active under normal operation serving local users, the CAS/HUB server at SITE B would serve as the FSW for DAG 2.

CAS/HUB

2 content filtering mail relays exist at SITE A delivering mail to the CAS/HUB at SITE A and 1 content filtering mail relay exists at SITE B delivering mail to the CAS/HUB at SITE B, all the relays are internet facing MX hosts for domains hosted in the exchange organization.

I expect in this configuration that WAN link failures would have no effect except to delay replication and that mailbox servers will be highly available failing over automatically. Client access failover will be manual and delayed by DNS TTL on A records. Clients in SITE A and B would be using different 5 minute TTL A records for access, each pointing to the local CAS under normal operation. The A records would resolve to internal addresses when using internal name servers and external addresses when using external name servers.

Questions:

-thoughts?

-what are my options for outbound mail routing? I assume that I'm able to specify different smart hosts for different HUB servers?

-is it recommended to have mailbox servers failover manually or automatically in this configuration?

-is there a better way to deal with client access failover?

-would a complete failure of all exchange servers at one site cause the passive copy at the alternate site to not be automatically activated? If so how much manual intervention would be required to force the passive copy to active if the FSW and normally active mailbox server go down? Would setting an alternate witness server fix this?

Thanks.
 
S

Sembee [MVP]



I see nothing wrong with what has been planned, although I wouldn't have Outlook pointing directly to the CAS servers, I would still be setting up a Client Access Array, even if it is only pointing to one server rather than a NLB cluster or similar. It will make life much easier for you and should be configured before you put any Outlook clients on to it. Then if you have to switch the CAS server for any reason (second server, replacement server, failover etc), doing so is very easy. If you introduce a CAS array later, or have to change the CAS server, then Outlook has to be touched to get it to update. Whereas using a CAS array means you just change the DNS and wait a few minutes.

If you lose everything in the other site, then I would expect the passive database to fail to come up automatically. This is actually a good thing, because the most likely reason is going to be a failure of the WAN. No WAN is guaranteed to be up 100% of the time. While that isn't a physical loss of the services, as far as the other side is concerned it is identical to that scenario.

For this very reason I would not want any automated way of the second site coming up on its own when access to the FSW is lost as that would cause a split brain scenario.

Simon.

Simon Butler, Exchange MVP
Blog | Exchange Resources
 
J

James-Luo



Q: “would a complete failure of all exchange servers at one site cause the passive copy at the alternate site to not be automatically activated? If so how much manual intervention would be required to force the passive copy to active if the FSW and normally active mailbox server go down? Would setting an alternate witness server fix this?”

Yes. Alternative witness server is used for such disaster

Datacenter Switchovers

Resources:

DAG Alternate Witness Server

James Luo

 
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