The appliances you select for the deployment must all be the same model, running the same software version. Otherwise, management and troubleshooting can become impractical.
Your appliance choice is generally made by comparing your site’s WAN bandwidth and number of WAN users to the capacities of the different appliances in the SD-WAN Data Sheet. For fault tolerance, always order one more appliance than is absolutely required according to the data sheet.
The number of appliances you need is found as follows, rounding up all fractions:
appliances = max ( appliances_bw, appliances_users ),
appliances_bw = ( WAN_bandwidth / Optimized_WAN_capacity ) +1
appliances_users = ( WAN_users / Maximum_HDX_sessions ) +1
Note that if appliances = 2, you can use just a single appliance instead of WCCP clustering, or an HA pair instead of WCCP clustering, since the equation builds in a spare appliance. In other words, WCCP clustering is not necessary (from a capacity perspective) unless appliances is 3 or more.
Example. Suppose you have 700 users and a 100 Mbps link. Some appliances you might consider are the SD-WAN 2000-050, the SD-WAN 3000-100, and the SD-WAN 4000-310.
|Model||Optimized WAN Capacity||Maximum HDX Sessions||Appliances_bw||Appliances_users||Appliances|
As you can see from the above table, the higher-performance platforms require fewer appliances to get the job done, as you would expect. The SD-WAN 4000-310 meets the requirements with a single appliance, and evaluates to two appliances only because the equations build in a spare.
You can always add more capacity by adding more appliances, but that is not always necessary. The bandwidth limits of two of the three choices, the SD-WAN 3000-100 and the SD-WAN 4000-310, can be increased through a license upgrade. The SD-WAN 2000-050 however, is already at the high end of the range for SD-WAN 2000 appliances.