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FIGURE 11-13 Client connectivity to the Client Access server during a mailbox copy failover
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FIGURE 11-14 Client connectivity to the Client Access Server during a Client Access server failover
Availability Planning for Transport Servers
Within the Exchange organization, it is important to deploy multiple transport servers to provide message path redundancy. Deploying multiple Hub Transports in each Active Directory site automatically provides redundancy and load balancing for message delivery. Deploying multiple Edge Transport servers will also provide incoming and outgoing SMTP redundancy.
Shadow redundancy
Exchange Server 2010 includes the shadow redundancy feature, which provides redundancy for messages for the entire time they are in transit. This is in addition to the transport dumpster. With one form of shadow redundancy, the message deletion from the transport queue is delayed until the transport server verifies that all of the next hops for that message have completed delivery. If any of the next hops fail before reporting successful delivery, the transport server resubmits the message for delivery to that next hop. If the next hop server does not support shadow redundancy, the message will be sent to the next hop and a shadow copy of the message will not be retained.
Availability Planning for Transport Servers
ChapTEr 11
Shadow redundancy provides the following benefits:
It eliminates the reliance on the state of the transport server queues. If redundant message paths exist, the state of any transport server isn t relevant. If a transport server fails, you can simply remove it from production without worrying about emptying its queues or losing messages currently in transit. If maintenance needs to be performed on the transport server the server can be brought offline without the risk of losing messages in transit. It reduces the need for hardware redundancy for transport servers for messages in transit. It consumes less bandwidth than other forms of redundancy that create duplicate copies of messages on multiple servers. With shadow redundancy the only added network traffic is the discard status being communicated between transport servers. It provides resilience and simplifies recovery from a transport server failure because messages still in transit within the Exchange organization are protected by the previous Exchange 2010 transport server.
Shadow redundancy does not protect messages in the transport dumpster,
importaNt
which is essential in being able to recover messages in the case of a DaG member failure.
One form of shadow redundancy is implemented by extending the SMTP protocol. These service extensions allow SMTP hosts to negotiate shadow redundancy support and communicate the discard status for shadowed messages. The protocol implementation of shadow redundancy works between Exchange 2010 transport servers. In the following scenario, a message is sent from an Exchange 2010 mailbox out to the Internet from a Hub Transport through an Edge Transport server, as shown in Figure 11-15. In this case the message flow follows these stages:
Hub delivers the message to Edge1:
a. Hub opens an SMTP session with Edge1. b. Edge1 advertises shadow redundancy support. c.
Hub notifies Edge1 to track discard status.
d. Hub submits the message to Edge1. e. Edge1 acknowledges receipt of the message and registers Hub1 to receive
discard information for the message.
f. 2.
Hub moves the message to the shadow queue for Edge1 and marks Edge1 as the primary server. Hub becomes the shadow server.
Edge1 delivers the message to the next hop:
a. Edge1 submits message to a third-party e-mail server. b. The third-party e-mail server acknowledges the message s receipt. c.
Edge updates the discard status for the message as delivery complete.
ChapTEr 11
Designing High Availability
If the message is delivered successfully, when Hub queries Edge1 for discard status:
a. At end of each SMTP session with Edge1, Hub queries Edge1 for the discard
status on messages previously sent. If Hub has not sent any other messages to Edge1, it will open an SMTP session with Edge1 to query for the discard status after five minutes and will fail over three failures or 15 minutes. This time can be configured using Set-TransportConfig with the ShadowHeartbeatTimeoutInterval parameter. The number of retries can be configured by running Set-TransportConfig -ShadowHeartbeatRetryCount.
b. Edge1 checks the local discard status and sends back the list of messages registered
to Hub1 that have been delivered and then removes the discard information.
c. 4.
Hub deletes the delivered messages from its shadow queue.
If the message delivery fails, then Hub queries Edge1 for discard status and resubmits the message:
a. If Hub cannot contact Edge1, Hub resumes the primary role and resubmits
the messages in the shadow queue to another available transport server, Edge2.
b. The resubmitted messages are delivered to Edge2, and the workflow starts from step 1.
2 Edge1
Hub 4 External SMTP Mail Server Edge2
FIGURE 11-15 Transport shadow redundancy
The Shadow Redundancy Manager (SRM) is the core component of a Transport server responsible for managing shadow redundancy. The SRM is responsible for maintaining the shadow server for all of its primary messages. The SRM is also responsible for maintaining the following information for all the shadow messages in its shadow queues:
Determining when the shadow server should take ownership of shadow messages, thus making it the primary server Maintaining the list and checking primary server availability for each shadow message
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