This is the interface number of the next-hop interface used by this manually-configured co-routed tunnel, identifying the specific local port the LSP egresses through rather than just the neighbor's IP address. An admin cross-references this against the device's interface table when the next-hop IP alone is ambiguous, such as when multiple parallel links exist to the same neighbor. For example, an interface number pointing to ifIndex 518 tells the admin exactly which physical port to check first if this tunnel's next hop appears unreachable, rather than checking every link to that neighbor.