Forwarding Known Unicast Frames
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To decide whether to forward a frame, a switch uses a dynamically built table that lists MAC
addresses and outgoing interfaces. Switches compare the frame’s destination MAC address to
this table to decide whether the switch should forward a frame or simply ignore it. For example,
consider the simple network shown in Figure 7-3, with Fred sending a frame to Barney.
Figure 7-3 Sample Switch Forwarding and Filtering Decision
In this figure, Fred sends a frame with destination address 0200.2222.2222 (Barney’s MAC
address). The switch compares the destination MAC address (0200.2222.2222) to the MAC
address table, matching the bold table entry. That matched table entry tells the switch to
forward the frame out port F0/2, and only port F0/2.
NOTE A switch’s MAC address table is also called the switching table, or bridging table,
or even the Content-Addressable Memory (CAM) table, in reference to the type of physical
memory used to store the table.
A switch’s MAC address table lists the location of each MAC relative to that one switch.
In LANs with multiple switches, each switch makes an independent forwarding decision
based on its own MAC address table. Together, they forward the frame so that it eventually
arrives at the destination.
For example, Figure 7-4 shows the first switching decision in a case in which Fred sends a
frame to Wilma, with destination MAC 0200.3333.3333. The topology has changed versus
the previous figure, this time with two switches, and Fred and Wilma connected to two
different switches. Figure 7-3 shows the first switch’s logic, in reaction to Fred sending the
original frame. Basically, the switch receives the frame in port F0/1, finds the destination
MAC (0200.3333.3333) in the MAC address table, sees the outgoing port of G0/1, so SW1
forwards the frame out its G0/1 port.
Figure 7-4 Forwarding Decision with Two Switches: First Switch
That same frame next arrives at switch SW2, entering SW2’s G0/2 interface. As shown in
Figure 7-5, SW2 uses the same logic steps, but using SW2’s table. The MAC table lists the
forwarding instructions for that switch only. In this case, switch SW2 forwards the frame
out its F0/3 port, based on SW2’s MAC address table.
Figure 7-5 Forwarding Decision with Two Switches: Second Switch
NOTE The forwarding choice by a switch was formerly called a forward-versus-filter
decision, because the switch also chooses to not forward (to filter) frames, not sending the
frame out some ports.
The examples so far use switches that happen to have a MAC table with all the MAC
addresses listed. As a result, the destination MAC address in the frame is known to the
switch. The frames are called known unicast frames, or simply known unicasts, because the
destination address is a unicast address, and the destination is known. As shown in these
examples, switches forward known unicast frames out one port: the port as listed in the
MAC table entry for that MAC address.