Switching Introduction
©2008 Allied Telesis Inc. All rights reserved.
14.8
AlliedWare Plus
TM
Operating System Software Reference C613-50003-00 REV E
Software Version 5.2.1
The Learning Process
The learning process uses an adaptive learning algorithm, sometimes called
backward learning
,
to discover the location of each station on the extended LAN.
All frames admitted by the ingress rules on any port are passed on to the forwarding process
when they are for destinations in the same VLAN. Frames destined for other VLANs are
passed to a Layer 3 protocol, such as IP. For every frame admitted, the frame’s source MAC
address and VID are compared with entries in the forwarding database for the VLAN (also
known as a
MAC Address table
) maintained by the switch. When the frame’s source address is
not in the forwarding database for the VLAN, the address is added and an ageing timer for that
entry is started. When the frame’s source address is already in the forwarding database, the
ageing timer for that entry is restarted.
By default, switch learning is enabled. It can be disabled with the
no mac address-table acquire
command, and re-enabled using the
mac address-table acquire command on page 15.7
.
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 with information about stations that are
inactive or have been disconnected from the network. It also ensures that entries for active
stations are kept alive in the forwarding database.
By default, the ageing timer is enabled. It can be disabled with the
no mac address-table
ageing-time
command. You can enable it with the
mac address-table ageing-time command on
page 15.7
.
If switch learning is disabled and the ageing timer has aged out all dynamically learned filter
entries, only statically entered MAC source addresses decide the packets to forward or discard.
When the switch finds no matching entries in the forwarding database during the forwarding
process, all switch ports in the VLAN are flooded with the packet, except the port that
received it.
The default for setting the mac address-table ageing-time is 300 seconds (5 minutes) and can
be modified by using the command
mac address-table ageing-time
.
To set the mac address-table ageing-time to 1000 seconds
To display general switch settings, including settings for switch learning and the switch ageing
timer, use the
show system command on page 8.26
.
The Forwarding Process
After a VID is assigned to a frame using the ingress rules, the switch forwards it to the
destination MAC address specified in the frame. To do this the switch must learn which MAC
addresses are available on each port for each VLAN. When the destination MAC address is not
found, the switch floods the frame on all ports that are members of the VLAN except the port
on which the frame was received.
The forwarding database (also known as the
MAC Address table
) determines the egress port on
which the destination MAC address has been learned. MAC addresses are learned dynamically
as part of the layer two switching process.
awplus#
configure terminal
Enter the config terminal mode
awplus(config)#
mac address-table
ageing-time 1000
Set the ageing time to 1000 seconds
