Azure: ARM template for bandwidth utilization alert for VPN gateways

 Part of having a good setup in Azure and in the Hub specifically is to have proper monitoring configured. In this article will be described a bandwidth utilization alert for VPN gateways using a static threshold as well as an action group that sends emails to a specified address.

In addition to having resource health alerts configured on relevant resources (which will let you know if resources become unavailable), it is also beneficial to know if key components are reaching or exceeding their capacity. This can be done using utilization type alerts.

The default setting if you add a tunnel bandwidth alert for a VPN gateway is dynamic thresholds. But unless you have a fairly consistent usage pattern, then we've seen that too many alerts are thrown which generates unnecessary noise (it might be a bit better if sensitivity is changed to low as opposed to the default medium setting).

Instead it makes sense to configure the alert using a static threshold. What that threshold should be depends on your specific setup. But it could be e.g. 75% or 90% of SKU capacity.

The VPN SKU capacity is specified in Gbps and the threshold is defined in bytes so you have to make that conversion. Below are some examples of how to calculate using SI / decimal system for bits and binary system for bytes (though I actually think that MSFT is using SI system for bytes as well - but it is not overly important in this context).

Examples are for a threshold of 2,5 Gbit for a 3 Gbps gateway and 0,9 Gbit for a 1 Gbps gateway:

• 1 Gbit = 1.000.000.000 bits
• 1 byte = 8 bits
• 1 Gbit = (1.000.000.000 / 8) = 125.000.000 bytes
  
  
• 2,5 Gbit = (125.000.000 * 2,5) = 312.500.000 bytes
• 0,9 Gbit = (125.000.000 * 0,9) = 112.500.000 bytes
  
  
• 1 Kilobyte = 2^10 bytes = 1024 bytes
• 1 Megabyte (MB) = 2^20 bytes = 1024 * 1024 bytes = 1.048.576 bytes
  
  
• 1 Gbit = (125.000.000 / 1.048.576) = 119,2 megabytes
• 2,5 Gbit = (312.500.000 / 1.048.576) = 298 megabytes
• 0,9 Gbit = (112.500.000 / 1.048.576) = 107,3 megabytes

The ARM template deploys alerts for two VPN gateways with two different thresholds. The VPN gateways must be deployed in advance (or you can adjust the template to use just one GW).

Only the parameter files needs to be updated with relevant info.

ARM template is available on Github:

util-alert-static-threshold-vpn-gw.json

util-alert-static-threshold-vpn-gw.parameters.json

The template contains an action group and it has two email addresses configured, this can be reduced to just one if needed.