Agree with the above info. I'll add that the validation checker Philip mentions kicks in when there are detected overlapping configured routes.
Example
Template A
- VLAN 1 a /24 from a 10.0.0.0/8
Template B
- VLAN 1 a /24 from a 10.0.0.0/8
Nothing is wrong there and dashboard allows the config as well as creates non overlapping child subnets. If however on a MX hub (concentrator mode) you had a local network (static route) for 10.0.0.0/8 that creates a conflict. In this case when you attempt to create the 10.0.0.0/8 template supernet it should stop you with the alert that no subnets are left.
Specifically the error is "Failed to auto-generate 1 unique subnet for VLAN '1': subnet pool '/24 from 10.0.0.0/8' has 0 subnets available."
If using BGP on the hub and not the "local networks" config the error doesn't occur.
As mentioned by another here it's best to design appropriately in the first place and not create overlaps. This also helps with the auto summarization that takes place in templates. So, if for example you had two templates using an approach like this for example.
Template A
- VLAN 1 a /24 from a 10.0.0.0/9
Template B
- VLAN 1 a /24 from a 10.128.0.0/9
This ensures no overlaps plus it allows auto summarization to take place when/where possible.
