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1-12 C
HAPTER
1: I
NTRODUCTION
DUA1860-0AAA01
Learning, Filtering And Forwarding
Transparent
bridges remain transparent to the network segments,
treating them as one overall network. The main operations of a
transparent bridge are
learning
,
filtering
and
forwarding
. These
operations are 802.1 bridge features and enable it to control the
flow of traffic to each network segment.
Devices send information as frames. The two main types of frame
are 802.3 and Ethernet. The destination address and source
address are contained within the frame, as shown in Figure 1-10
.
Figure 1-10 Frame Contents
Every time the bridge receives a packet, it looks at the source
address and destination address. If the bridge has not previously
received a packet on that port from the device, it learns the
source address by adding it to a list of device addresses connected
to the port. The bridge then compares the destination address to
the address lists for all the ports on the bridge. If the destination
address appears on the address list of a port that did not receive
the packet, the bridge
forwards
(duplicates) the packet to that
port. If the destination address appears on the address list of the
same port that received the packet, the bridge
filters
(discards)
the packet. If the destination address does not appear on any of
its address lists, the bridge passes it on to all but the receiving
port, called
flooding
.
Destination
Address
802.3 frame
Ethernet frame
Destination
Address
Length
Type
Data
Data
Frame
Checksum
Frame
Checksum
Source
Address
Source
Address