Friday, July 13, 2012

Lync Edge and RevProxy behind Link Controller

Normally, if a customer has high available Internet connections and the Providers are supporting a proper load balancing, they will make use of BGP4 (http://en.wikipedia.org/wiki/Border_Gateway_Protocol) Border Gateway Protocol. If this is the case, you have a straight forward setup for Lync Edge Server and RevProxy.

But what's happen if BGP is not available?
There is only one solution, you need to establish the load balancing by yourself. How we are doing this is by simple introducing a Link Controller. Now we are able to provide a high available and failover solution for outgoing Internet Traffic .
But incoming traffic is still not load balanced. How should we do this?

Ok, you need to play with external DNS entries!
What does this mean? A DNS entry can have multiple A Record, e.g. we are using for Lync DNS based Load Balancing. Within the Round Robin procedure, e.g. you have two DNS A Records, every second DNS query the other IP Address will be provided.

meet.contoso.com A 12.15.15.1
meet.contoso.com A 202.12.16.1

But here a problem will occur on the Internet. every DNS entry has TTL (Time To Live), if now and sure it is like this in the Internet wide world DNS, DNS caching is mostly active. Therefore Round Robin breaks off and if one Client request two time the DNS name, it will be provided with the same IP.
That not what we want!
We need to cheat here a little bit.

If you have a Windows based DNS Server, you need to change under VIEW to ADVANCED, than edit both of your DNS A Record.
Change the Time to live to the shortest period you can provide, but it hast to be support by your Internet Provider.



Now back to LYNC.
As I described, LYNC EDGE Server cannot be used with this technology. But it's only half the truth, you can still make use for the RevProxy.
I have drawn a general design how you can implement this technology. First, and don't forget about this, ask your Internet Provider about "HIDDEN DNS" configuration. Here you configure your own DNS Server and let the MASTER DNS from your Provide poll your ZONE configuration periodically.


Hope this is interesting. Not really complex, but possible and workable. You can also add a high weight for Internet Connection II, e.g. 30/70, this would ensure Lync has more bandwidth available.
Truly this is not ha fully high availability solution, but better than nothing.

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