Ahmad,
The network will still function, but it will not be optimized. By having traffic go Spoke-Hub-Spoke instead of Spoke-Spoke, you could experience the following:
- Bandwidth Saturation: With a large number of spokes, the Hub’s bandwidth could become saturated with all the traffic that could be going between the spokes directly
- Increased delay: Applications that are sensitive to delay (like voice or video) might experience adverse effects because of the extra hops and introduced delay with Spoke-Hub-Spoke traffic
- Increased dependence on the Hub being available: Even through the Hub will remain a single point of failure for the control plane (meaning the spokes still depend on the hub to learn about spoke to spoke routes), once a spoke establishes spoke to spoke communication, the hub could go down for a short time and not interrupt the traffic between spokes (that have already learned of each other). With Spoke-Hub-Spoke data flow, this means that the Hub is also in the data plane of the traffic, so if the hub has any interruption in service, even for a moment, then spoke to spoke traffic will suffer.