Forwarding/Filtering Decision

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
MAC address table
SYST RPS
STRT UTIL DUPLXSPEED
MODE
1 2 3 4 5 6 7 8 9 10 11 12
10Base-T/100Base-TX Catalyst 2950 SERIES
Layer 2 LAN Switching
485
on the correct exit interface. The switch does not transmit the frame out of any interface except
for the destination interface, thus preserving bandwidth on the other network segments. This is
called frame filtering.
If the destination hardware address is not listed in the MAC database, the frame is flooded out
all active interfaces except the interface on which the frame was received. If a device answers, the
MAC database is updated with the device location (interface).
In modern switches, the switching or bridging table is known as the CAM or
TCM table. I will cover these in detail in Chapter 21, “Catalyst Switch Technologies.”
For the moment, please accept that these are just tables, optimized for
pretty fast lookup.