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Configuring the Router to 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 router IS-IS advertisements are originated with the overload bit set. This causes the transit traffic to avoid the overloaded router and take paths around the router. However, the overloaded router's own links are still accessible.

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

You can configure the local router so that it appears to be overloaded. You might want to do this when you want the router 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 router.) To mark the router as overloaded, include the overload statement:




overload {
advertise-high-metrics;
timeout seconds;
}

To advertise maximum link metrics in NLRIs instead of setting the overload bit, include the advertise-high-metrics option when specifying the overload statement:



advertise-high-metrics;

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



overload timeout seconds;

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.

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