How switches learn hosts’ locations

How switches learn hosts’ locations
In the figure, four hosts are attached to a switch. The switch has nothing in the MAC address
table when it is powered on. The figure shows the switch’s MAC filter table after each device
has communicated with the switch. The following steps show how the table is populated:
1.
Station 1 sends a frame to station 3. Station 1 has a MAC address of 0000.8c01.1111.
Station 3 has a MAC address of 0000.8c01.3333.
2.
The switch receives the frame on Ethernet interface 0/1, examines the source and destination
MAC addresses, and places the source address in the MAC address table.
3.
Because the destination address is not in the MAC database, the frame is forwarded out of
all interfaces; this is called flooding.
4.
Station 3 receives the frame and responds to station 1. The switch receives this frame on
interface E0/3 and places the source hardware address in the MAC database.
5.
The switch knows the interface associated with station 1’s MAC address and forwards the
frame only out of that interface. Effectively, station 1 and station 3 now have a point-topoint
connection, and only those two devices will receive the frames. Stations 2 and 4 do
not see the frames.
If the two devices do not communicate with the switch again within a certain time limit, the
switch flushes the entries from the database to keep the database as current as possible.
Forwarding/Filtering Decision
The layer 2 switch also uses the MAC filter table to both forward and filter frames received on
the switch. This is called the
forwarding and filtering decision
. When a frame arrives at a switch
interface, the destination hardware address is compared to the forward/filter MAC database. If
the destination hardware address is known and listed in the database, the frame is sent out only
E0/1
E0/3
E0/2
E0/4
0000.8c01.1111 0000.8c01.2222
0000.8c01.3333 0000.8c01.4444
1
3
2
4
Station 1 sends a frame to station 3.
Destination is known; frame is not flooded.
E0/1: 0000.8c01.1111
E0/2: 0000.8c01.2222
E0/3: 0000.8c01.3333
E0/4: 0000.8c01.4444