Suppose an r40 has set up two interfaces eth4 and eth5. eth4 is in subnet A, eth5 is in subnet B, and default gateway is in A. Now, if the monitoring point is monitoring a target that is reachable from both networks, it would always take the path through A. So how do we get monitoring to go through B for a specific target? Simply add a second interface, and the path configuration wizard then allows you to select a source interface for the path.

Upon changing the source interface to eth5, the monitoring point immediately routes monitoring traffic for the path over eth5 while all other paths remain on eth4. Behind the scenes, what’s happening is the source monitoring point adds a route table for the source interface and it is always consulted before the main routing table. You can see these routes in web admin.

Known issue on m22, m30, r40, r400: There is a special case for interfaces with dhcp-assigned addresses. When a path is configured using an interface which later becomes unavailable—because the interface was deleted, shutdown, or the IP address changed—the path will enter the 50x50_trans.png connectivity lost state. Under 50x50_trans.png > Configure the interface will be marked ‘missing’. If you save a path config that uses a missing interface the connectivity check will pass, but the subsequent diagnostic will hang.