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17.1.2 Forward or Filter
The switch will forward or filter received data frames according to the MAC table. Take the
above figure as an example, assuming switch have learnt the MAC address of PC1 and PC3,
and the user manually configured the mapping relationship for PC2 and PC4 to ports. The
MAC table of switch will be:
MAC Address
Port number
Entry added by
00-01-11-11-11-11
1/5
Dynamic learning
00-01-22-22-22-22
1/5
Static configuration
00-01-33-33-33-33
1/12
Dynamic learning
00-01-44-44-44-44
1/12
Static configuration
Forward data according to the MAC table
If PC1 sends a message to PC3, the switch will forward the data received on port 1/5 from
port1/12.
Filter data according to the MAC table
If PC1 sends a message to PC2, the switch, on checking the MAC table, will find PC2 and
PC1 are in the same physical segment and filter the message (i.e. drop this message).
Three types of frames can be forwarded by the switch:
Broadcast frame
Multicast frame
Unicast frame
The following describes how the switch deals with all the three types of frames:
Broadcast frame: The switch can segregate collision domains but not broadcast domains. If no
VLAN is set, all devices connected to the switch are in the same broadcast domain. When the
switch receives a broadcast frame, it forwards the frame in all ports. When VLANs are
configured in the switch, the MAC table will be adapted accordingly to add VLAN information.
In this case, the switch will not forward the received broadcast frames in all ports, but forward
the frames in all ports in the same VLAN.
Multicast frame: For the unknown multicast, the switch will broadcast it in the same vlan, but
the switch only forwards the multicast frames to the multicast group’s port if IGMP Snooping
function or the static multicast group has been configured.
Unicast frame: When no VLAN is configured, if the destination MAC addresses are in the
switch MAC table, the switch will directly forward the frames to the associated ports; when the
destination MAC address in a unicast frame is not found in the MAC table, the switch will
broadcast the unicast frame. When VLANs are configured, the switch will forward unicast
frame within the same VLAN. If the destination MAC address is found in the MAC table but
belonging to different VLANs, the switch can only broadcast the unicast frame in the VLAN it
belongs to.
Summary of Contents for QSW-2800 series
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