Why didn't my routing work?

You are concerned that your routing isn't working because you sent a packet, but it didn't go through.

Sometimes packets go through with no issues, but other times the packets fail to arrive as expected.

It is possible that you have experienced an asymmetric routing issue, where packets take one path from the source to the destination, but replies follow a different return path.

Asymmetric routing isn't always a problem, but it can cause issues when something in your packet is stateful. For example:

• The initial TCP connection (SYN) establishes a path through the Software Defined Network (SDN).
• If the initial SYN passes through a firewall-router, a stateful route is established for the connection through the firewall-router.
• The TCP acknowledgement (SYN, ACK) and all future packets for the connection must follow the path of the stateful connection.

Packets are dropped when the return path takes a route that is different then the initial SYN.

Force traffic in both directions through the same firewall-router.

For more information about asymemtric routing solutions, see Centralize communication through a VPC Transit Hub and Spoke architecture - Part one and Centralize communication through a VPC Transit Hub and Spoke architecture - Part two.