Specifications

Switching 8-29
Software Release 2.7.3
C613-03098-00 REV A
If the ageing timer for an entry in the forwarding database expires before
another frame with the same source address is received, the entry is removed
from the forwarding database. This prevents the forwarding database from
being filled up with information about stations that are inactive or have been
disconnected from the network, while ensuring that entries for active stations
are kept alive in the forwarding database. The ageing timer is enabled by
default; it can be disabled or enabled by using the commands:
ENAble SWItch AGEingtimer
DISable SWItch AGEingtimer
If switch learning is disabled and the ageing timer has aged out all dynamically
learned filter entries, only statically entered MAC source addresses are used to
decide which packets to forward or discard. If the switch finds no matching
entries in the forwarding database during the Forwarding Process, then all
switch ports in the VLAN are flooded with the packet, except the port on
which the packet was received.
The default of the ageing timer is 300 seconds (5 minutes) but can be modified
by using the command:
SET SWItch AGEingtimer=10..1000000
The forwarding database relates a station’s (source) address to a port on the
switch, and is used by the switch to determine from which port to transmit
frames with a destination MAC address matching the entry in the station map.
To display the contents of the forwarding database, use the command:
show switch fdb [address=macadd] [discard={source|
destination}] [hit={yes|no}] [l3={yes|no}]
[port={portlist|all}] [status={static|dynamic}]
[vlan={vlan-name|1..4094}]
To display general switch settings, including settings for switch learning and
the switch ageing timer, use the command:
show switch
The Forwarding Process
The forwarding process forwards received frames that are to be relayed to
other ports in the same VLAN, filtering out frames on the basis of information
contained in the station map and on the state of the ports. When a frame is
received on the port for a destination in a different VLAN, it is either Layer 3
switched if it is an IP packet, or looked up in the Layer 3 routing tables.
Forwarding occurs only when the port on which the frame was received is in
the Spanning Tree forwarding or disabled states. The destination address is
then looked up in the forwarding database for the VLAN. If the destination
address is not found, the switch floods the frame on all ports in the VLAN
except the port on which the frame was received. If the destination address is
found, the switch discards the frame if the port is not in the STP forwarding or
disabled states, if the destination address is on the same port as the source
address, or if there is a static filter entry for the destination address set to
discard (see “Layer 2 Filtering” on page 8-31). Otherwise, the frame is
forwarded on the indicated port.
This whole process can further be modified by the action of static switch filters.
These are configurable filters that allow switched frames to be checked against
a number of entries.