3Com 9100 manual Forwarding Database FDB

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FORWARDING DATABASE (FDB)

This chapter describes the contents of the forwarding database (FDB), how the FDB works, and how to configure the FDB.

Overview of the The switch maintains a database of all media access control (MAC)

FDBaddresses received on all of its ports. It uses the information in this database to decide whether a frame should be forwarded or filtered.

FDB Contents 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.

FDB Entry Types 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.

■Non-aging entries — If the aging time is set to zero, all aging entries in the database are defined as non-aging entries. This means that they do not age, but they are still deleted if the switch is reset.

■Permanent entries — Permanent entries are retained in the database if the switch is reset or a power off/on cycle occurs. The system administrator must make entries permanent. A permanent entry can either be a unicast or multicast MAC address. All entries entered by