This read-write INTEGER selects how the tunnel interface's local address is determined: configured by the user, automatic (device's minimum IP address), or interface (minimum IP address on a specified interface). An admin setting up a tunnel needs to choose the right source method depending on whether they want a fixed address or one that adapts automatically as the device's addressing changes. For example, choosing interface(3) and pointing it at a specific stable management interface avoids the tunnel's source address changing if a lower-numbered but less stable interface gets an even smaller IP.