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Configuring IS-IS to Make Routing Devices Appear Overloaded

If the time elapsed after the IS-IS instance is enabled is less than the specified timeout, overload mode is set.

You configure or disable overload mode in IS-IS with or without a timeout. Without a timeout, overload mode is set until it is explicitly deleted from the configuration. With a timeout, overload mode is set if the time elapsed since the IS-IS instance started is less than the specified timeout.

A timer is started for the difference between the timeout and the time elapsed since the instance started. When the timer expires, overload mode is cleared. In overload mode, the routing device IS-IS advertisements are originated with the overload bit set. This causes the transit traffic to avoid the overloaded routing device and take paths around the routing device. However, the overloaded routing device’s own links are still accessible.

To summarize, the value of the overload bit depends on these three scenarios:

1. When the overload bit has already been set to a given value and the routing process is restarted: LSPs are regenerated with the overload bit cleared.
2. When the overload bit is reset to a lesser value while the routing process is running: LSPs are regenerated with the overload bit cleared.
3. When the overload bit is reset to a greater value while the routing process is running: LSPs are regenerated with the overload bit set to the difference between the old and new value.

In overload mode, the routing device advertisement is originated with all the transit routing device links (except stub) set to a metric of 0xFFFF. The stub routing device links are advertised with the actual cost of the interfaces corresponding to the stub. This causes the transit traffic to avoid the overloaded routing device and take paths around the routing device. However, the overloaded routing device’s own links are still accessible.

You can configure the local routing device so that it appears to be overloaded. You might want to do this when you want the routing device to participate in IS-IS routing, but do not want it to be used for transit traffic. (Note that traffic to immediately attached interfaces continues to transit the routing device.) To mark the routing device as overloaded, include the overload statement:

To advertise maximum link metrics in network layer reachability information (NLRI) instead of setting the overload bit, include the advertise-high-metrics option when specifying the overload statement:

When you configure the advertise-high-metrics option, the routing device in overload mode stops passing (leaking) route information into the network. So an L1-L2 routing device in overload mode stops passing route information between Level 1 and Level 2 and clears its attached bit when the advertise-high-metrics option is configured.

To allow route information to pass (leak) into the network when the routing device is in overload mode, include the allow-route-leaking option when specifying the overload statement:

Note: The allow-route-leaking option does not work if the routing device is in dynamic overload mode. Dynamic overload can occur if the device has exceeded its resource limits, such as the prefix limit.

To specify the number of seconds at which overload is reset, include the timeout option when specifying the overload statement:

The time can range from 60 through 1800 seconds.

For a list of hierarchy levels at which you can include these statements, see the statement summary sections for these statements.