Overload bit

If an IS‘s resources are overloaded and are preventing the IS from properly performing IS-IS routing, the IS can inform other ISs of this condition by setting the overload bit in LSPDUs sent to other ISs from 0 (off) to 1 (on). Thus, when an IS is overloaded, other ISs will not use the overloaded IS to forward traffic.

An IS can be in the overload state for Level 1 or Level 2 as follows:

• If an IS is in the overload state for Level 1, other Level 1 ISs stop using the overloaded IS to forward Level 1 traffic. However, the IS can still forward Level 2 traffic, if applicable.
• If an IS is in the overload state for Level 2, other Level 2 ISs stop using the overloaded IS to forward Level 2 traffic. However, the IS can still forward Level 1 traffic, if applicable.
• If an IS is in the overload state for both levels, the IS cannot forward traffic at either level.

By default, the device automatically sets the overload bit to 1 (on) in its LSPDUs to other ISs if an overload condition occurs.

The overload bit can also be set to administratively shut down IS-IS without disabling the protocol. Setting the overload bit on is useful when configuration changes are needed but you do not want to remove the device from the network. A device can also be configured to set the overload bit on for a specific number of seconds during startup, to allow IS-IS to become fully active before the device begins IS-IS routing.