The overload (OL) field of LSPs configured on a device prevents other devices from calculating the routes passing through this device.
If IS-IS enters the Overload state after an exception occurs on the device, the system deletes all imported or leaked routes.
If IS-IS enters the Overload state based on a user configuration, the system only deletes all imported or leaked routes if configured to do so.
Although LSPs with overload fields are flooded throughout the network, they are ignored in the calculation of the routes passing through the device in the Overload state. Specifically, after the overload field of LSPs is configured on a device, other devices do not count the routes that pass through the device when performing SPF calculation, but the direct routes between the device and other devices are still calculated.
If a device in an IS-IS domain is faulty, routes may be incorrectly calculated across the entire domain. The overload field can be configured for the device to isolate it from the IS-IS network temporarily, which facilitates fault isolation.