2 cissp ® Official Study Guide Eighth Edition

Bridges
A
bridge
is used to connect two networks together—even networks of different 
topologies, cabling types, and speeds—in order to connect network segments that use the 
same protocol. A bridge forwards traffi c from one network to another. Bridges that con-
nect networks using different transmission speeds may have a buffer to store packets until 
they can be forwarded to the slower network. This is known as a
store-and-forward
device. 
Bridges operate at OSI layer 2. Systems on either side of a bridge are part of the same 
broadcast domain but are in different collision domains. 
Switches
Rather than using a hub, you might consider using a switch, or intelligent hub. 
Switches
know the addresses of the systems connected on each outbound port. Instead of 
repeating traffi c on every outbound port, a switch repeats traffi c only out of the port on 
which the destination is known to exist. Switches offer greater effi ciency for traffi c delivery, 
create separate collision domains, and improve the overall throughput of data. Switches 

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Chapter 11 
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Secure Network Architecture and Securing Network Components
can also create separate broadcast domains when used to create VLANs. In such configura-
tions, broadcasts are allowed within a single VLAN but not allowed to cross unhindered 
from one VLAN to another. Switches operate primarily at OSI layer 2. When switches have 
additional features, such as routing, they can operate at OSI layer 3 as well (such as when 
routing between VLANs). Systems on either side of a switch operating at layer 2 are part of 
the same broadcast domain but are in different collision domains. Systems on either side of 
a switch operating at layer 3 are part of different broadcast domains and different collision 
domains. Switches are used to connect network segments that use the same protocol.