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Deploying VLANs 93
Routing Technologies
Routing is a key technology for connecting LANs in a campus network. Routers interconnect
at Layer 3 of the OSI model. A router's main job is to filter or forward packets. The device looks
at the IP address of the destination packet and determines whether the packet should be
forwarded. Routers use routing protocols to share information about routes and destinations.
Routing can be achieved by the use of Layer 3 switching or with traditional routers. Routers
add functionality beyond bridges. Routers create collision and bandwidth domains. Routers
prevent broadcasts from propagating across networks.
Routers provide the following functionality:
·
Segment LANs and WANs
·
Determine the best path to the destination based on the network layer protocol
·
Communicate route information with other routers
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Send the data to other routers using the best path
Considerations for Scaling a Switched Network
The number of workstations that can be used on a flat network depends on the protocol:
·
Avoid having more than 500 IP stations on a flat network.
·
When using AppleTalk and NetBIOS, use no more than 200 stations.
·
When using IPX, use no more than 300 stations.
Networks that rely heavily on broadcasts and multicast protocols should limit the maximum
number of workstations.
Deploying VLANs
When a switch divides up a broadcast domain, a virtual LAN (VLAN) is created. A VLAN
divides a single domain into several broadcast domains. A VLAN can be configured as a logical
group of end stations with a common set of requirements. VLANs allow users in any location
to participate in any VLAN. Switches can be connected to create a trunk. With a trunk, users
can be separated by miles and still participate in the same VLAN.
VLANs allow logical network topologies to overlay the physical switched infrastructure such
that any arbitrary collection of LAN ports can be combined into an autonomous user group or
community of interest. The technology logically segments the network into separate Layer 2
broadcast domains, whereby packets are switched between ports designated to be within the
same VLAN. By containing traffic originating on a particular LAN to the same VLAN,
switched virtual networks avoid wasting bandwidth, a drawback inherent in traditional bridged
87200333.book Page 93 Wednesday, August 22, 2001 2:18 PM