Extreme Networks ExtremeWare XOS 11.3, Руководство пользователя

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Forwarding Database
ExtremeWare XOS 11.3 Concepts Guide
250
How FDB Entries Get Added
Entries are added into the FDB in the following ways:
●
The switch can learn entries by examining packets it receives. The system updates its FDB with the
source MAC address from a packet, the VLAN, and the port identifier on which the source packet is
received.
The ability to learn MAC addresses can be enabled or disabled on a port-by-port basis.
You can also limit the number of addresses that can be learned, or you can “lock down” the current
entries and prevent additional MAC address learning.
NOTE
For more information on port control for learning MAC address, refer to Chapter 5 .
●
You can enter and update entries using the command line interface (CLI).
●
Certain static entries are added by the system upon switch boot-up.
FDB Entry Types
FDB entries may be dynamic or static, and the entries may be permanent or non-permanent. The
following describes the types of entries that can exist in the FDB:
●
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. 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 may 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
show fdb
output.
Dynamic entries age—that is, a dynamic entry is removed from the FDB (aged-out) if the device
does not transmit for a specified period of time (the aging time). This aging process prevents the
FDB 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. For more information
about setting the aging time, see
“Configuring the FDB Aging Time” on page 252 .