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Chapter 5
Forwarding Database (FDB)
Overview of the FDB
FDB Contents
FDB Entry Types
This chapter describes the contents of the forwarding database
(FDB), how the FDB works, and how to configure the FDB.
The switch maintains a database of all media access control (MAC)
addresses received on all of its ports. It uses the information in this
database to decide whether a frame should be forwarded or filtered.
The database holds up to a maximum of 128K entries. Each entry
consists of the MAC address of the device, an identifier for the port
on which it was received, and an identifier for the VLAN to which the
device belongs. Frames destined for devices that are not in the FDB
are flooded to all members of the VLAN.
The following are three types of entries in the FDB:
Dynamic entries — Initially, all entries in the database are
dynamic. 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. Dynamic entries are deleted from the database if
the switch is reset or a power off/on cycle occurs. For more
information about setting the aging time, refer to the section
"Configuring FDB
Entries," later in this chapter.
5-1
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