Layer 2 Switching | 45 |
ENABLE SWITCH LEARNING
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. By default, the ageing timer is enabled, and it can be disabled or enabled 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 will be 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 will be flooded with the packet, except the port on which the packet was received.
The default value of the ageing timer is 300 seconds (5 minutes), and this can be modified 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 (if any) 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={SOURCEDESTINATION}] [HIT={YESNO}] [L3={YESNO}] [PORT={portlistALL}] [STATUS={STATICDYNAMIC}] [VLAN={vlanname1..4094}]
To display general switch settings, including settings for switch learning and the switch aging 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. If 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 (see the Rapier Switch Software Reference.)
Forwarding occurs only if the port on which the frame was received is in the Spanning Tree ‘Forwarding’ state. 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’ state, 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 (“Layer 2 Filtering” on page 46). Otherwise, the frame is forwarded on the indicated port.
Rapier Switch Software Release 2.2.1
