3.5.h Implement, troubleshoot and optimize EIGRP convergence and scalability

3.5.h [v] Summary metric

When EIGRP creates a summary route, it includes a metric with the route in order to advertise it. EIGRP searches for components of the summary to be suppressed and represented by the summary. EIGRP finds the component with the best metric and copies the metric from it into the summary. Components of the summary may come and go, which means that every time the best component changes, the summary needs to be re-advertised to all of its peers. Even if the best component is not the one that changed, EIGRP still has to search every topology entry to make sure the summary is not affected.

This can add significant processing overhead. Use the summary-metric command to mitigate this metric churn and processing overhead. Rather than searching for the best component metric, EIGRP uses the values configured with the summary-metric command.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3279-3286).  . Kindle Edition.

http://www.cisco.com/c/en/us/td/docs/ios/iproute_eigrp/command/reference/ire_book/ire_s1.html#wp1059226