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When a prefix is included in the most-active list for any single snapshot period, it will be written into the trending database. If there are no archives for storing data for that prefix, new database files will be created.
If a prefix subsequently falls out of the most-active list, new data will not be stored, but the archives that were created for it will still exist until you clear them with the clear trending or trending clear-after (deprecated) command.
When you select Prefix in the Type menu on the Trend Definition page, the list of prefixes displayed in the Select Prefix box represents all prefixes for which the trending database has tables.
Some prefixes may not be among the most-active very often, and the trending database may therefore contain very little data for them. Because of the way the graphing process uses averaged values when there are insufficient datapoints, graphlines based on measurements for these little-used prefixes may appear to be empty because of this lack of data gathering.
Prefixes that are of interest to you should be added to a report group. Trending data is gathered for any prefix that is in a group.
NOTE: You can create or add to a group from the web UI by selecting a group of prefixes in a snapshot report or show prefixes display and clicking the right mouse button. Enter the report group’s name--either an existing group or a new group name--and click the OK button. The prefixes will be added to the specified group in the device’s running configuration. Or, from the CLI, use the report group command to create a report group or add a prefix to an existing group. (Be sure to save your running config to a disk location or to the startup config if you want the report group to survive a system reload.)
Use the trending clear-after (deprecated) CLI command to prune little-used prefixes from the database.
Some gaps in the data may still occur, however, due to system down time, or breaks in peerings with edge routers. When these gaps occur, there will be corresponding gaps in the graphlines.
See the discussion of Prefix in the section Trend Report Data Types elsewhere in this chapter for more information about which prefixes will have database files.
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