This value is the tunnel identifier assigned to a manually configured static MAC address entry, specifying which NVGRE tunnel traffic to that MAC should be encapsulated into. Administrators set this when creating a static MAC mapping to ensure traffic to a specific remote host always uses a known, predictable tunnel rather than relying on dynamic learning. For example, an admin might statically map a critical server's MAC address to tunnel ID 5 to guarantee its traffic always takes that specific overlay path.