Exchange 2013 coexistence environment and the Exchange legacy infrastructure | 8#23

of 21 | Part 08#23 | Exchange 2013 coexistence environment and the Exchange
legacy infrastructure
Written by Eyal Doron | o365info.com | Copyright © 2012-2015
EXCHANGE 2013 COEXISTENCE
ENVIRONMENT AND THE EXCHANGE LEGACY
INFRASTRUCTURE | 8#23
In this article, I would like to focus on the term “legacy” that is used very often in the
Exchange 2013 coexistence environment. As usual, the common assumption is that
everyone knows and understands this term, but in reality, the term is not so clear,
and we are as ashamed to admit that we don’t fully understand the meaning of the
term.

In the context of Exchange 2013 coexistence environment, the term: “Legacy” can
be translated into a couple of options:
 Legacy Exchange client
 Legacy Exchange infrastructure
 Legacy namespace
Using the term legacy – not a politically-
correct term
In my opinion, the term “legacy” seems always a little out of place.
What is the meaning of legacy?
Does it mean that, up until now, the existing Exchange infrastructure that was
sworn to “serve and protect” is useless or, not good anymore?
Does it mean that only the “new Exchange infrastructure” is meaningful and
important?

I know it’s philosophizing but, I think that the “Exchange guys”, should choose a
more politically-correct term because, the association that appears in mind when I
hear the term “legacy” is something like this:
Regardless of my reservations, as a loyal citizen of the Exchange nation, I will
continue to use this term as long as it needed.

How to introduce Exchange CAS 2013 to
existing Exchange environment
Let’s make it simple. When a new Exchange server version is released, and we want
to “adapt” this new Exchange server version we have two basic options:
Option 1: “Wipe out” the existing Exchange infrastructure and “plant” the new
Exchange server version or in more technical words: install the new Exchange
server version and decommission the former Exchange server version.
Option 2: Implement an Exchange infrastructure that can be described as: “side by
side” which mean: add the new Exchange server version (Exchange 2013 server, in
our case) to the existing Exchange environment and, slowly start to “migrate” all the
resources such as user mailboxes and Exchange web services to the “new Exchange
infrastructure”.

Only after there is no Exchange client that relies onuse the former Exchange
infrastructure, we can start to decommission the “older Exchange environment”.
“Old” Exchange infrastructure | Terms used
When implementing a project of Exchange coexistence, the Exchange 2013
infrastructure is the “new infrastructure” and all the rest of the former Exchange
server versions can be described as:
 Older version of Exchange
 Former versions of Exchange
 Legacy Exchange infrastructure
In the following diagram, we can see that from the Exchange 2013 point of view,
Exchange 2010 infrastructure and Exchange 2007 infrastructure consider as: Legacy
Exchange infrastructure.

Legacy Exchange clients
The term “legacy Exchange client” relates to Exchange client that their mailbox is
hosted on the legacy Exchange mailbox server.
In our scenario of Exchange 2013 coexistence environment, each Exchange user
who is a mailbox is hosted on Exchange 2007 Mailbox server or Exchange 2010
Mailbox server consider as “legacy Exchange client”.

Throughout the series of articles, I will not use the term: “legacy Exchange client”
but instead, I will refer to “non-Exchange CAS 2013 server client” as: Exchange 2007
clients or Exchange 2010 clients.
In an Exchange 2013 coexistence environment, “legacy Exchange clients” such as
Exchange 2007/2010 clients will need to Initialize the communication process with
the Exchange 2013 CAS server, and the “rest” of the process will be “determined” by
the Exchange 2013 CAS.
Exchange CAS 2013 will decide how to continue the client protocol connectivity flow
based the specific scenario parameters such as the Exchange client version, the
Exchange client physical location and so on.
For example, the Exchange CAS 2013 can decide to Proxy the legacy Exchange client
request to the legacy Exchange infrastructure by himself or send to the legacy
Exchange client a redirection command, etc.

How the Exchange CAS 2013 server does treat
his legacy clients?
Q1: “Why does the Exchange 2013 server need to deal with legacy Exchange clients?
Why not implementing an architecture, in which Exchange 2013 will serve only
Exchange 2013 client, and the “other legacy Exchange servers” will serve their
legacy Exchange clients?

A1: Well, the simplified answer is that in an Exchange 2013 coexistence
environment, the most updated Exchange server version (Exchange 2013 CAS in
our scenario) will need to be configured as the “focal point” for all the Exchange
clients, including native Exchange 2013 “native clients” + legacy Exchange clients
(Exchange 2007/2010 clients).
Note – You can read more information about the concept of Exchange CAS 2013
server as a focal point in the article – Exchange Public infrastructure | Public versus
non Public facing Exchange site
The Exchange 2013 CAS is “smart enough” to recognize Exchange client version
such as Exchange legacy client and decide about the “next step” such as: Proxy the
Exchange legacy client requites to the “right Exchange legacy infrastructure”
For example, when Exchange 2007 Outlook client (Exchange user who is a mailbox
is hosted on Exchange 2010 Mailbox server) address Exchange CAS 2013 server, the
Exchange CAS 2013 server “understand” that the specific client “belong” to the

Exchange 2010 infrastructure and for this reason, the Exchange CAS 2013 server
will proxy the request to available Exchange 2007 CAS.
Note – from the Exchange 2007 client point of view, this process is transparent. The
meaning is that the Exchange 2007 client is not where to the fact that his request
was “routed” to the Exchange 2007 CAS.
From the Exchange 2007 client point of view, the Exchange CAS 2013 server is the
element that provides the required mailbox data.
The reason for using Legacy namespace in
Exchange 2013/2007 coexistence environment
Let’s start from the end: the term “Exchange legacy namespace”, is relevant only in
a scenario of Exchange 2013/2007 coexistence environment.
The use of the “Exchange legacy namespace”, will be implemented for Exchange
2007 OWA clients and Exchange 2007 Outlook clients that access the Exchange
2007 web services.

Exchange 2007 clients and request for access mailbox content
In Exchange 2013/2007 coexistence environment, Exchange 2013 CAS behaves
differently with different Exchange 2007 client type.
 In a scenario of Exchange 2007 Outlook and ActiveSync client that request access
to their mailbox content, Exchange CAS 2013 will proxy connection request to the
Exchange CAS 2007.
 In a scenario in which the Exchange 2007 client is OWA client, Exchange CAS 2013
will not use the Proxy method, but instead, redirect the Exchange 2007 OWA
client to the Exchange CAS 2007 by using the Exchange CAS 2007 legacy
namespace.
This is one of the two reasons behind the need to create and use the legacy
namespace in Exchange 2013/2007 coexistence environment.

Exchange CAS 2013 server, doesn’t know how to proxy, Exchange 2007 OWA client
requests to Exchange 2007 CAS server.
Generally speaking, Exchange CAS 2013 server prefers to implement the proxy
method because the Proxy method makes the Exchange client life simple. The
Exchange client doesn’t need to be familiar with the complex infrastructure of the
client protocol connectivity flow or play an “active role” in the communication
process. When implementing the method of Proxy, Exchange CAS 2013 does all the
“hard work,” work” and the Exchange client just enjoys from the ability to access his
mailbox data.
Because the Exchange CAS 2013 server doesn’t know how to proxy, Exchange 2007
OWA clients’ request, Exchange 2013 CAS uses a method which described as:
redirect or if we want to be more specific: silent redirect + SSO.

Note – You can read more information about silent redirect + SSO in the article
– OWA client protocol connectivity flow in Exchange 2013/2007 coexistence
environment | 3/4
The redirection method is implemented by sending the Exchange client a URL
address of “other Exchange CAS servers” that can help the Exchange client.
In a scenario of Exchange 2007 OWA mail client, the Exchange CAS 2013 server will
send the client the URL address of existing Exchange 2007 CAS server.

To be able to “refer” the Exchange 2007 OWA client to the Exchange CAS 2010, the
Exchange CAS 2013 “send” the Exchange 2007 OWA client browser a redirection
command with the URL address of the “destination Exchange CAS 2017” that
include the Exchange CAS 2007 legacy namespace.
The term: legacy namespace, describe the namespace that is assigned or “attached”
to the Exchange 2007 CAS server. For example: legacy.mail.o365info.com
In other words, to be able to refer to other or additional Exchange CAS servers (the
Exchange CAS 2007) we will need to use a different namespace form the primary
namespace. This is the reason or the need for using the: “legacy namespace”.
Exchange 2007 Exchange web services clients
The second reason for using the legacy namespace in Exchange 2013/2007
coexistence environment is the subject of: providing Exchange web services to
Exchange 2007 clients.
In Exchange 2013/2007 coexistence environment, Exchange 2007 client will need to
get their Exchange web services from Exchange 2007 CAS and not from Exchange
2013 CAS.
The reason is Exchange 2013 CAS doesn’t know how to provide Exchange web
services to the Exchange 2007 clients.
In the following diagram, we can see the concept of “redirecting Exchange 2007
clients” requests for Exchange web services to the – Exchange 2007 CAS.
In reality, there is no “real redirection process.
The Autodiscover information that Exchange 2013 CAS provides to the Exchange
2007 clients, includes the URL address and the FQDN that represents the legacy
namespace of the Exchange 2007 CAS such as: legacy.mail.o365info.com
When Exchange 2007 client looks at the host who can provide him Exchange web
services, Exchange 2007 client will use the URL address which includes the “referral”
to the Exchange CAS 2007.

Legacy names space – The concept of a
uniquededicated namespace
In Exchange 2013/2007 coexistence environment, Exchange CAS 2013 need to
1. Redirect To Exchange 2007 OWA clients to the Exchange CAS 2007 server.
2. “Inform” Exchange 2007 client that the Exchange web service will be provided by
the Exchange CAS 2007 server.
To be able to “refer” to the Exchange CAS 2007, we will need to use a
uniquededicated namespace for the Exchange CAS 2007 infrastructure, which is
different from the primary namespace.

The “additional namespace” that will be assigned to the Exchange CAS 2007 server
described as: “legacy” because as we mention, Exchange relates to former versions
of Exchange server as: legacy.
The legacy namespace is based on a concept in which we continue to use the
existing domain name and add or use a different host name (prefix).
For example: in our scenario, the domain name is: o365info.com
The primary namespace that is used for publishing and addressing the Exchange
CAS 2013 is: mail.o365info.com
To be able to differentiate the Exchange CAS 2007 from the “primary namespace”,
we can use a naming convention such as: legacy.mail.o365info.com or another
option is to add to host name: “legacy” to the existing primary namespace. For
example: legacy.mail.o365info.com

Technically speaking, there is no mandatory need for using the host name: legacy.
The legacy is not a reserve host name but instead, just a common naming
convention.
For example: we can use the following namespace: unclesam.mail.o365info.com as
the legacy namespace for Exchange CAS 2007 server.
It’s important to mention that will need to “publish” and update the “new legacy
namespace” is different infrastructures.
Recap

We can use the following diagram as a “recap” for what we have to learn up until
now.
 The concept of Exchange legacy namespace is relevant only in an Exchange
2013/2007 coexistence environment.
 The “clients” that will be used the legacy namespace are: Exchange 2007 OWA
clients + Exchange 2007 clients such as Outlook that use the Exchange 2007 as a
source server for Exchange web service.
 Exchange CAS 2013 uses the legacy namespace for implementing the process of:
“silent redirection” in which send a redirection command to the Exchange 2007
client browser using the legacy Exchange CAS 2007 namespace.
 Exchange CAS 2013 uses the legacy namespace as part of the Autodiscover
information that he provides to Exchange 2007 clients. The Exchange 2007 client
will use the URL address that includes the Exchange 2007 legacy namespace for
addressing Exchange CAS 2007 when they need Exchange web services.

The concept of two Public facing Exchange
CAS server in a single Exchange site
At first look, the concept of: “two Public facing Exchange CAS server in a single
Exchange site” seems a little odd. The standard convention of Exchange public
infrastructure in a scenario of: a Public facing Exchange site, is implemented most
of the time, by using a specific Exchange CAS server who serves as the
“representative” of a Public facing Exchange site.
Even in a scenario in which we implement a fault tolerance and load balancing
mechanism and use an array of Exchange CAS servers, the external client “see” only
one entity or one namespace such as: mail.o365info.com
In a scenario of Exchange 2013/2007 coexistence environment, we need to
implement a configuration, in which we need to use: two Public faces Exchange CAS
servers at the same time.
Exchange CAS 2013 which will be the “main Public facing Exchange CAS server” and
additionally, an Exchange CAS 2007 which will also have to have a public availability.
The “Exchange 2007 Public facing server” will serve Exchange 2007 OWA clients.

The scenario in which external Exchange 2007 clients, will address the Public facing
Exchange 2007 CAS server are as follows:
1. External Exchange 2007 OWA clients
Exchange 2007 OWA clients will address the “primary” or the “main” Public facing
Exchange CAS server meaning – the Exchange CAS 2013. When the Exchange CAS
2013 recognizes that the external client is: Exchange 2007 OWA client, the Exchange
CAS 2013 will start a process of silent redirection in which he redirects the
Exchange 2007 OWA client browser, to the “Exchange 2007 Public facing server”.
For example:
Exchange 2007 OWA client addresses the Public facing Exchange 2013 CAS server
by using the URL address: mail.o365info.com
The redirection command that will “refer” the Exchange 2007 client, to the
Exchange 2007 Public facing server is based on a URL address that includes the
legacy namespace of the Public facing Exchange 2007 server. For example, the URL
address could be: https://legacy.mail.o365info.com/owa
The Exchange 2007 OWA client, will need to
 Find the public IP address of the host: legacy.mail.o365info.com
 Need to create a new HTTPS session with the “new destination host” the Public
facing Exchange 2007 server.
In simple words, to be able to enable this “flow”, we will need to “expose” also the
Exchange 2007 CAS and configure the Exchange 2007 server as a Public facing
Exchange CAS server.
2. External Exchange 2007 Outlook clients
Exchange 2007 Outlook clients will address the “primary” or the “main” Public facing
Exchange CAS server as the Autodiscover Endpoint. When the Exchange CAS 2013
recognizes that the external client is: Exchange 2007 Outlook client, the Exchange
CAS 2013 will create a “custom Autodiscover answer”, which includes the URL
address of the “Exchange 2007 Public facing server”.

When Exchange 2007 Outlook clients need a specific web service, the client will
address the Public facing Exchange 2007 CAS server using the legacy namespace.
The Exchange 2013 coexistence article series index page

Exchange 2013 coexistence environment and the Exchange legacy infrastructure | 8#23

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