Information Technology Reference
In-Depth Information
Layer 3 IP address and adds a new Layer 2 address at the egress interface. In addition to
controlling collision domains, routers bound data link layer broadcast domains. Each inter-
face of a router is a separate broadcast domain. Routers do not for ward data link layer
broadcasts. IP defines network layer broadcast domains with a subnet and mask. Routers
are aware of the network protocol, which means they can forward packets of routed pro-
tocols, such as IP and IPX. Figure 3-5 shows a router; each interface is a broadcast and a
collision domain.
Broadcast Domain
Broadcast Domain
Collision Domain
Collision Domain
Figure 3-5
Routers Control Broadcast and Collision Domains
Routers exchange information about destination networks using one of several routing
protocols. Routers use routing protocols to build a list of destination networks and to
identify the best routes to reach those destinations. The following are examples of routing
protocols:
Enhanced Interior Gateway Routing Protocol (EIGRP)
■
Open Shortest Path First (OSPF)
■
Border Gateway Protocol (BGP)
■
Routing Information Protocol (RIP)
■
Intermediate System-to-Intermediate System (IS-IS)
■
Chapters 10 and 11 cover routing protocols in further detail. Routers translate data-link
protocols. They are the preferred method of forwarding packets between networks of dif-
fer ing media , s uch as Ether net to Token Ring or Ether net to s er ial. They als o prov ide
methods to filter traffic based on the network layer address, route redundancy, load bal-
ancing, hierarchical addressing, and multicast routing.
Layer 3 Switches
LAN switches that can run routing protocols are
Layer 3 switches
. These switches can
run routing protocols and communicate with neighboring routers. They are also referred
to as multilayer switches. Layer 3 switches have LAN technology interfaces that perform
network layer packet forwarding. The use of switching technologies at the network layer
greatly accelerates packet forwarding between connected LANs, including VLANs. You
can use the router capacity you save to implement other features, such as security filtering
and intrusion detection.
Layer 3 switches perform the functions of both data link layer switches and network layer
