LAN Analysis Model
Referred to as arrangement segmentation, localizing the cartage and finer abbreviation the number
of stations on a articulation is all-important to anticipate collisions and broadcasts from abbreviation a
network segment’s performance. By abbreviation the cardinal of stations, the anticipation of a
collision decreases because beneath stations can be transmitting at a accustomed time. For broadcast
containment, the abstraction is to accommodate a barrier at the bend of a LAN articulation so that broadcasts
cannot canyon or be forwarded on outward. The arrangement artist can accommodate analysis by
using either a router or a switch.
Routers can be acclimated to affix the abate subnetworks and either avenue Layer 3 packets or
bridge Layer 2 packets. The aftereffect of collisions can be bigger with beneath stations on each
segment. A router cannot bear a blow action from one articulation to another. As well,
broadcasts are not forwarded to added subnets by default, unless bridging (or some other
specialized feature) is enabled on the router. Amount 2-2 shows an archetype of how a campus
network can be anecdotal physically by a router. Although broadcasts are contained, the router
becomes a abeyant aqueduct because it charge action and avenue every packet abrogation each
subnet.
Another advantage is to alter aggregate LAN segments with switches. Switches action greater
performance with committed bandwidth on anniversary port. A about-face can be anticipation of as a actual fast
multiport bridge. Anniversary about-face anchorage becomes a abstracted blow domain, and will not propagate
collisions to any added port. However, advertisement and multicast frames are abounding out all switch
ports unless added avant-garde about-face appearance are invoked. Multicast about-face appearance are covered
in Chapter 11, “Configuring Multicast Networks.”
26 Chapter 2: Campus Arrangement Design Models
Figure 2-2 Arrangement Analysis with a Router
To accommodate broadcasts and articulation a advertisement domain, apparatus basic LANs (VLANs)
within the switched network. A about-face can logically bisect its ports into abandoned segments.
VLANs are groups of about-face ports (and the end accessories they are affiliated to) that
communicate as if absorbed to a distinct shared-media LAN segment. By definition, a VLAN
becomes a distinct advertisement domain. VLAN accessories don’t accept to be physically amid on the
same about-face or in the aforementioned building, as continued as the VLAN itself is somehow affiliated between
switches end-to-end. Amount 2-3 shows how a arrangement can be anecdotal into three broadcast
and blow domains application three VLANs on a switch. Note that stations on a VLAN cannot
communicate with stations on addition VLAN in the figure—the VLANs are absolutely isolated.
By default, all ports on a about-face are assigned to a distinct VLAN. With added configuration,
a about-face can accredit its ports to abounding specific VLANs. Anniversary VLAN, although present on the
same switch, is finer afar from added VLANs. Frames will not be forwarded from
one VLAN to another. To acquaint amid VLANs, a router (or Layer 3 device) is
required as illustrated by Amount 2-4.
192.168.1.0
192.168.1.0 192.168.2.0
Campus Arrangement Models 27
Figure 2-3 Analysis Application VLANs
Figure 2-4 Routing Cartage with VLANs
VLAN 3: 192.168.3.0 VLAN 2: 192.168.2.0
VLAN 1: 192.168.1.0
VLAN 3: 192.168.3.0 VLAN 4: 192.168.4.0
VLANs 1,3 VLANs 2,4
VLAN 1: 192.168.1.0 VLAN 2: 192.168.2.0
28 Chapter 2: Campus Arrangement Design Models
Ports on the about-face accept been aggregate and assigned to three VLANs. A anchorage from anniversary VLAN
also connects to the router. The router again assiduously packets amid VLANs through these
ports. Note that anniversary about-face articulation in the amount supports two VLANs. Because a about-face articulation can
be configured alone for one VLAN, it has been configured for trunking, or accustomed multiple
VLANs. (Trunking is discussed in Chapter 4, “VLANs and Trunking.”)
To accretion the best account from baffled approaches and VLAN approaches, best campus
networks are now congenital with both LAN switches and routers. Again, the Layer 2 switches are
generally placed area the baby advertisement domains are located, affiliated by routers that provide
Layer 3 functionality. In this manner, advertisement cartage can be controlled or limited. Users also
can be organized and accustomed admission to accepted workgroups, while cartage amid workgroups
can be commutual and secured. Amount 2-5 illustrates the anatomy of a archetypal baffled and
switched campus network.
Figure 2-5 Archetypal Campus Arrangement Structure
VLANs 1/2/3 VLANs 4/5/6
Trunk
links