I am in the middle of planning a fairly large migration and one issue has been kind of bugging me. It kept gnawing at me. I was waking up in the middle of the night and just can’t explain why.
DNS Load Balancing (DNS LB) is a new feature in Lync Server 2010. Basically, DNS LB uses DNS round-robin entries, and the Lync 2010 clients use their own mechanism to load balance across servers. The Lync client is able to use the entries in DNS and use its own algorithm to select a server from the servers in a pool. If the selected server is unavailable, the Lync client is able to select another server in the pool. There are lots of great articles on how DNS LB works, so I won’t go into any depth here.
What keeps bugging me is which comes first in a migration, the chicken or the egg? By this, I mean, do you deploy new servers first or new clients first. Well, that is easy in the case of an upgrade to Lync. The Lync 2010 client is not supported for use with Office Communications Server 2007 R2 (OCS 2007 R2) servers. The server environment must be upgraded first.
Wait. What does that mean?
OK, the basic migration from OCS 2007 R2 to Lync Server 2010 goes like this (I am going to ignore the Edge for now, as it doesn’t matter for my point):
- Start with Microsoft Office Communicator 2007 R2 (MOC 2007 R2) clients and OCS 2007 R2 servers.
- Add a new Lync 2010 Server pool(s).
- Migrate users to the new pool(s).
- Upgrade MOC 2007 R2 clients to Lync 2010 clients.
- Retire OCS 2007 R2 servers.
Yes, there are many more steps for the migration, but these are the big ones. Did you notice that after Step3, you have MOC 2007 R2 clients running against Lync Server 2010 servers?
So what? Well, in order to use DNS LB, you must have Lync 2010 clients. So, how do you get the MOC 2007 R2 clients to run against the Lync Server 2010 server environment? Well, you do that by not using DNS LB. You have to set up Hardware Load Balancing (HLB) if you want MOC 2007 R2 clients to be able to properly connect to Lync Server 2010 servers as the MOC 2007 R2 clients are not able to leverage DNS LB and still support high availability (HA) while performing the migration.
So, to summarize: Unless you can do a green field installation of Lync Server 2010 or can cut over every single client from MOC 2007 R2 to the new Lync 2010 client at once, the dream of DNS LB won’t be reality. After all, if you need to configure HLBs to support your HA requirements during the migration, why would you bother going back and implementing DNS LB?