The bandwidth requirements remain the same as they were when 8x was released. However, the round trip time (RTT) requirements have changed from 150 ms to 60 ms. Which is quite a big jump. It looks like this change was made on August 29, 2013.
This is more than a little disconcerting because I was looking at the design guide in July and made design recommendations based on the previous RTT requirement. Fortunately, my customer's network can still accommodate the updated budget. But what if that wasn't the case and I had to make a major design change in the middle of the project? Or worse, what if I didn't go back to re-read the design guide and the customer ran into an issue?
Basically, I would have been screwed. Again, I am fortunate that we are still within budget and that I generally pick the lowest common denominator for clustering over the WAN designs. Which, prior to August 29, 2013, was 80 ms RTT for CUCM clustering. Interestingly enough, UCCX clustering also has a 80 ms RTT budget. It is odd (to me) that Unity would drop below the 80 ms threshold.
Last point of interest. The UC 9x SRND still states that the maximum Round Trip Time (RTT) budget is 150 ms. That is probably still there to keep things interesting for the operators in the field! Obviously, it is best to err on the side of caution and assume the SRND recommendation is no longer valid.
I tried to find more information to see why the sudden drop in RTT budget. I suspect there may be a defect or some other revelation. If anyone has more information please post a comment. I am genuinely curious.
Thanks for reading. If you have time, post a comment!