Dynamic Entries

A dynamic entry is learned by the switch by examining packets to determine the source MAC address, VLAN, and port information.

The switch then creates or updates an FDB entry for that MAC address. Initially, all entries in the database are dynamic, except for certain entries created by the switch at boot-up.

Entries in the database are removed (aged-out) if, after a period of time (aging time), the device has not transmitted. This prevents the database from becoming full with obsolete entries by ensuring that when a device is removed from the network, its entry is deleted from the database.

The aging time is configurable, and the aging process operates on the supported platforms as follows:

On all platforms, you can configure the aging time to 0, which prevents the automatic removal of all dynamic entries.
On all platforms, the aging process takes place in software and the aging time is configurable.

For more information about setting the aging time, see Configuring the FDB Aging Time.

Note

If the FDB entry aging time is set to 0, all dynamically learned entries in the database are considered static, non-aging entries. This means that the entries do not age, but they are still deleted if the switch is reset.

Dynamic entries are flushed and relearned (updated) when any of the following take place:

A VLAN is deleted.
A VLAN identifier (VLANid) is changed.
A port mode is changed (tagged/untagged).
A port is deleted from a VLAN.
A port is disabled.
A port enters blocking state.
A port goes down (link down).

A non-permanent dynamic entry is initially created when the switch identifies a new source MAC address that does not yet have an entry in the FDB. The entry can then be updated as the switch continues to encounter the address in the packets it examines. These entries are identified by the “d” flag in the show fdb command output.