2 cissp ® Official Study Guide Eighth Edition

Routers Routers
are used to control traffic flow on networks and are often used to con-
nect similar networks and control traffic flow between the two. They can function using 
statically defined routing tables, or they can employ a dynamic routing system. There are 
numerous dynamic routing protocols, such as RIP, OSPF, and BGP. Routers operate at OSI 
layer 3. Systems on either side of a router are part of different broadcast domains and dif-
ferent collision domains. Routers are used to connect network segments that use the same 
protocol.
Brouters Brouters
are combination devices comprising a router and a bridge. A brouter 
attempts to route first, but if that fails, it defaults to bridging. Thus, a brouter operates 
primarily at layer 3 but can operate at layer 2 when necessary. Systems on either side of 
a brouter operating at layer 3 are part of different broadcast domains and different colli-
sion domains. Systems on either side of a brouter operating at layer 2 are part of the same 
broadcast domain but are in different collision domains. Brouters are used to connect net-
work segments that use the same protocol.
Gateways
A 
gateway
connects networks that are using different network protocols. A 
gateway is responsible for transferring traffic from one network to another by transform-
ing the format of that traffic into a form compatible with the protocol or transport method 
used by each network. Gateways, also known as 
protocol translators
, can be stand-alone 
hardware devices or a software service (for example, an IP-to-IPX gateway). Systems on 
either side of a gateway are part of different broadcast domains and different collision 
domains. Gateways are used to connect network segments that use different protocols. 
There are many types of gateways, including data, mail, application, secure, and internet. 
Gateways typically operate at OSI layer 7.