This is something that has happened on my home and work network. Perhaps you set up a personal website on a raspberry pie that is inside your home network. You get everything up and running, you have destination NAT or port forwarding configured on your home router. You buy a domain and set up A-record with your home’s internet IP. Everything is great and you can access your website from the internet....
Asymmetric routing means that the request packets are sent to one gateway, but the response packets are received from another gateway, that is, the returning packets are taking a different path. Normally this wouldn’t be problematic as the end devices don’t really care about which gateway they receive the packets from. However, asymmetric routes become a problem when NAT is used or having a firewall in between the network. When a firewall receives a response packet that it’s unaware of the original flow, it may consider the response packet invalid and drops it....