What if you have a supported back-end server version (e.g. OCS 2007 R1) to coexist with Lync, but you’re running Office Communicator 2005 out on the desktop? What happens when you want to migrate?
It’s going to be pretty rare that you might encounter this kind of scenario, but I did recently and it’s worth documenting the behaviour we observed.
Unfortunately there’s no pretty screenshots in this post, so use your imagination. 🙂

The Environment

The situation I encountered was an OCS 2007 R1 backend with all clients running Office Communicator 2005. We deployed a new Lync 2010 Front End pool in the same forest and the two pools happily talked to each other (as OCS 2007 R1 and Lync Server 2010 coexistence is supported).
It was only until we begin running client testing that we started to notice weird things happening.

The Behaviour

If you have one user using Office Communicator 2005 and another user using Lync 2010, you will see strange behaviour if the session is initiated from the Lync user to the OC 2005 user.
If the Lync 2010 user polls for presence of the OC 2005 user in any way, whether this is by searching the address book for a user, expanding a distribution group or exposing their name in Outlook, the OC 2005 user will get a Add to Contact List notification. This detracts from the way things usually work, where you can get presence of a user without first adding them to your contact list.
When I tested this same scenario using OC 2007 R1 and Lync 2010, I couldn’t reproduce the issue.

My only guess is that there is a difference between the SIP INFO packets Lync 2010 and OC 2007 R1/R2 send to get presence information, and the way OC 2005 interprets them.

Obviously this is not ideal, and will most likely pose an unacceptable issue during a period of coexistence whilst migrations are occurring. Given that hundreds of these interactions could occur a day per user in a large environment, this presents a massive support issue for your internal IT helpdesk.

The Solution

In the end, you have two options depending on how long your period of coexistence will be and how complicated/large your environment is. These are:

1. Migrate from OC 2005 to OC 2007 R1 on your existing OCS 2007 R1 backend, then migrate your users to the Lync 2010 backend and upgrade your clients to Lync 2010.
2. Migrate your users to the Lync 2010 backend and upgrade your clients to Lync 2010 all in one go, effectively having no period of coexistence.

In conclusion, TechNet documentation defines that this kind of coexistence is “only supported if Communicator 2005 is on a federated network” and now we know why. I’d say this has something to do with what the Edge Server does to the SIP packets on their way in and out of the network.