Configuring Office Web Apps Location in Lync 2013

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For those that have deployed Lync 2013 alongside Office Web Apps 2013, we’ve all seen this seemingly innocuous check box:

Office Web Apps Server is deployed in an external network (that is, perimeter/Internet)
Over the past year I’ve never yet had a reason to check that box and honestly didn’t have a very good idea of what happens if you do. Office Web Apps is still becoming learned by Lync integrators and administrators, so not much information is available on the Internet on what occurs when that option is selected. “Curiosity killed the cat”, they say, but I survived and have a better understanding of Lync and Office Web Apps as a result.
Office Web Apps Background
Generally speaking, each Office Web Apps farm has two URLs configured – an internal URL and an external URL. For example, my lab environment has the following configuration:

InternalURL – https://owapool01-intweb.widgets.com
ExternalURL – https://owapool01-extweb.widgets.com
Note: Technically speaking, the two URLs could be exactly the same. I would advise against that approach, however, as it requires having to extend split-brain DNS to Office Web Apps.  Additionally, having distinct URLs keeps in-line with the general URL best practices/requirements for Lync Web Services in having separate and distinct URLs for internal web services vs external web services.
Within my Lync topology, I have configured the topology to have a single Office Web Apps server:

The discovery URL above is what Lync uses to obtain information from Office Web Apps on what URLs are available for the Lync Web Conferencing MCU and Lync clients to use for PowerPoint broadcasting. When the Lync Web Conferencing MCU starts up and all is well between your Lync and OWA (WAC) environments, you’ll see the following entry within the Lync Application event log:

Web Conferencing Server Office Web Apps Server (WAC) discovery has succeeded
Office Web Apps Server internal presenter page: https://owapool01-intweb.widgets.com/m/Presenter.aspx?a=0&e=true&
Office Web Apps Server internal attendee page: https://owapool01-intweb.widgets.com/m/ParticipantFrame.aspx?a=0&e=true&
Office Web Apps Server external presenter page: https://owapool01-extweb.widgets.com/m/Presenter.aspx?a=0&e=true&
Office Web Apps Server external attendee page: https://owapool01-extweb.widgets.com/m/ParticipantFrame.aspx?a=0&e=true&
The message above indicates that Office Web Apps discovery has completed and Lync can successfully use OWA (WAC) for PowerPoint broadcasting. An important note here is that the URLs above are not maintained within the Central Management Store. Once Lync completes the OWA (WAC) discovery, the URLs returned by Office Web Apps are maintained within memory only. If you were to change the OWA (WAC) farm URLs, you would need to restart the Lync Web Conferencing service before the changes would become effective for your clients.
Office Web Apps Location
If your Lync topology currently does not have the Office Web Apps Server is deployed in an external network (that is, perimeter/Internet) check box checked, when a PowerPoint broadcast is started by a Lync 2013 client, the Lync Server 2013 Web Conferencing service (Data MCU) hands out one of the four URLs above to the clients and offers only the URL that corresponds with the client location. Thus if I am an internal user I will receive only information regarding the internal URL. If I am an external user I will receive only information regarding the external URL. This can be confirmed by looking at the client UCCAPI logs:
ClientType=Lync;Build=15.0.4420.1017;ContentMCU=”sip:ethel.a.merman@widgets.com;gruu;opaque=app:conf:data-conf:id:SBMTGTNK”;ConferenceUri=”sip:ethel.a.merman@widgets.com;gruu;opaque=app:conf:focus:id:SBMTGTNK”;LocalFqdn=”PIA-WIN7-WK01.widgets.com”;Url=”https://owapool01-intweb.widgets.com/m/Presenter.aspx?a=0&e=true&WopiSrc=https%3A%2F%2Flyncpool01-intweb.widgets.com%2FDataCollabWeb%2Fwopi%2Ffiles%2F2-1-30CDE7F&access_token=AAMFENCpa_d_ZPtQon_XE5zcakwGEKpKzTdM7apF2LHn4iRQGciBENCpa_d_ZPtQon_XE5zcakyCArMGgyBvFooflH4vHS0Ow9tvt8Tb3fISHpC-QYmWss9wIMfT2IYIG7KV9zAC0QgIDURhdGFDb2xsYWJXZWI&<fs=FULLSCREEN&><rec=RECORDING&><thm=THEME_ID&><ui=UI_LLCC&><rs=DC_LLCC&>”
In the example above, I can confirm my internal client was only offered the internal URL for the Office Web Apps farm.
If you were to alter your Lync topology and check the Office Web Apps Server is deployed in an external network (that is, perimeter/Internet) check box, when a PowerPoint broadcast is started by a Lync 2013 client, the Lync Server 2013 Web Conferencing service (Data MCU) hands out only the external URL to the clients regardless of the client location. Thus if I am an internal user I will receive only information regarding the external URL. If I am an external user I will still receive only information regarding the external URL. This can be confirmed looking at the client UCCAPI logs:
ClientType=Lync;Build=15.0.4420.1017;ContentMCU=”sip:ethel.a.merman@widgets.com;gruu;opaque=app:conf:data-conf:id:67J3YH9C”;ConferenceUri=”sip:ethel.a.merman@widgets.com;gruu;opaque=app:conf:focus:id:67J3YH9C”;LocalFqdn=”PIA-WIN7-WK01.widgets.com”;Url=”https://owapool01-extweb.widgets.com/m/Presenter.aspx?a=0&e=true&WopiSrc=https%3A%2F%2Flyncpool01-extweb.widgets.com%2FDataCollabWeb%2Fwopi%2Ffiles%2F3-1-1A504B0&access_token=AAMFEFoLcM7OXe1QhLU__5hjkeQGEKpKzTdM7apF2LHn4iRQGciBEFoLcM7OXe1QhLU__5hjkeSCAordgyD3ue87wIfbYutqnANrU5dD0g0Ntk4Pb_eP6XJqJf8rh4YIZTjBiDIC0QgIDURhdGFDb2xsYWJXZWI&<fs=FULLSCREEN&><rec=RECORDING&><thm=THEME_ID&><ui=UI_LLCC&><rs=DC_LLCC&&gt
End Result
The overall result isn’t all that earth-shattering, but should definitely be understood by Lync administrators.
If your OWA (WAC) servers are in a perimeter DMZ with only an external URL specified, then make sure you select this option. Doing so ensures that all clients, regardless of location, utilize the external URL. Additionally, make sure you have all appropriate DNS records in place within your internal DNS zones if you chose to utilize this option.
Note: Technically you could have an internal OWA (WAC) server with only an external URL specified, and as a result could utilize the option above. Either way, I would again caution against this as it goes against best practices of maintaining separate URLs for internal vs external services.
If your OWA (WAC) servers are internal with both an internal URL and external URL specified, then I do not recommend selecting this option. By leaving it off, you let Lync Server handle the overall logic of which URLs get offered to clients. This method ensures that Office Web Apps operates in the same manner as the normal Lync pool web services.