Names the local interface through which this tunnel's traffic is routed. On a gateway with multiple WAN uplinks, this tells the admin exactly which physical or logical interface a given site-to-site tunnel is currently bound to, which matters when one uplink is degraded and only tunnels associated with it need attention. Example: finding multiple tunnels all reporting the same degraded interface at the same time, while tunnels on a different interface are fine, narrows the problem to that specific uplink rather than to VPN configuration in general.