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Chapter Eight
Local Area Networks: Internetworking
Data Communications and Computer
Networks: A Business User’s Approach,
Fourth Edition
Introduction
• Many times it is necessary to connect a local
area network to another local area network or to
a wide area network
• Different ways on how different LANs are
(to p3)
connected
• Business Application Cases
(to p41)
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Connecting LANs
• 6 types of network interconnection hardware
are available: (to p4)
– see Figure 12.18
– a) Hub – use to connect workstations together
(to p6)
– b) bridges - use to expand LANs with same protocol
(to p20)
– c) switches- work faster since need no dest. address
(to p35)
– d) routers- OSI level 3, uses translate dest. address
– e) brouters - combination of router and bridges
(to p38)
– f) gateways - connect different protocol of LANs (to p39)
– g) use of backbone as a structure for data interchange
– h) use of LAN switch to connect to MAN or WAN (to p40)
(to p5)
(Note: Now we usually use low CPU computer as
part of these devices, if programmable)
(to p2)
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FIGURE 12-18
Network interconnection can occur at various layers of the OSI model.
(to p3)
hub
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Hubs
• Hub interconnects two or more workstations into
a local area network
• When a workstation transmits to a hub, hub
immediately resends the data frame out to all
connecting links
• Can be managed or unmanaged
– A managed hub possesses enough processing
power that it can be managed from a remote
location
(to p3)
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Bridges
• A bridge (or bridge-like device) can be used to connect
(to p7)
two similar LANs, such as two CSMA/CD LANs
• Can also be used to connect two closely similar LANs,
such as a CSMA/CD LAN and a token ring LAN
• Examines
– destination address in a frame and either forwards this
frame onto next LAN or does not
– source address in a frame and places this address in a
routing table, to be used for future routing decisions
• Two main types
(to p13)
(to p9)
– Transparent Bridge
– Remote Bridge (to p12)
• Spanning tree solution to installing too many bridges
when connection LANs together (Optional topic!)
(to p3)
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Bridges (continued)
(to p6)
or
(to p8)
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FIGURE 12-21
LANs interconnected with a backbone LAN.
(to p3)
(FDDI)
(to p6)
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Transparent Bridge
• A bridge observes each frame that arrives at a port,
extracts the source address from the frame, and places
that address in the port’s routing table
(to p10)
(to p11)
• A transparent bridge is found with CSMA/CD LANs
• Can also convert one frame format to another
– This does not happen too often anymore since most
networks are CSMA/CD
– Note that some people / manufacturers call a bridge such
as this a gateway or sometimes a router
• Removes headers and trailers from one frame format
and inserts (encapsulates) headers and trailers for the
second frame format
(to p6)
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Transparent Bridge (continued)
(to p9)
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Transparent Bridge (continued)
(to p9)
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Remote Bridge
• Capable of passing a data frame from one LAN
to another when the two LANs are separated by
a long distance and there is a WAN connecting
the two LANs
• Takes frame before it leaves the first LAN and (to p19)
encapsulates the WAN headers and trailers
– When the packet arrives at destination remote
bridge, that bridge removes WAN headers and
trailers leaving original frame
(to p6)
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Spanning Tree Algorithm
• What happens if you have many LANs
interconnected with multiple bridges, such as
(to p14)
shown in the next slide?
– Data that leaves one workstation could travel to a
bridge, across the next network, into the next
bridge, and back onto the first network
– A packet may continue to cycle like this forever!
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Spanning Tree Algorithm (continued)
(to p15)
Data confusing
areas
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Spanning Tree Algorithm (continued)
(to p16)
Solution method
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Spanning Tree Algorithm (continued)
• How do we stop this from happening?
– Disconnect one of the bridges?
• Maybe we want bridge redundancy in case one
bridge fails
– Apply the spanning tree algorithm?
• How is the algorithm applied?
(to p17)
(to p6)
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Spanning Tree Algorithm (continued)
• Step 1: Designate a root bridge
• Step 2: Mark one port of each bridge as the root port
– Root port is port with the least-cost path from that bridge to the
root bridge
(to p18)
– Root ports are denoted with a star in Figure 8-7(a)
• Step 3: Select a designated bridge for each LAN
– Designated bridge has the least-cost path between that LAN and
root bridge
– Mark the corresponding port that connects that LAN to its
designated bridge with two stars (Figure 8-7(b))
(to p18)
• Step 4: Remove redundant ports
– If port has no stars, that port is redundant and can be “removed”
– Keep all ports with one or two stars
(to p18)
• Resulting configuration is shown in Figure 8-7(c)
• Note there is now only one way to get to any LAN or bridge from any
other LAN or bridge
(to p16)
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Spanning Tree Algorithm (continued)
(to p17)
(to p17)
(to p17)
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Remote Bridge (continued)
(to p12)
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Switches
• A combination of hub and bridge
• Can interconnect two or more workstations, but
like a bridge, it observes traffic flow and learns
(to p22)
• When a frame arrives at a switch, switch
examines destination address and forwards
frame out the one necessary connection
• Workstations that connect to a hub are on a
shared segment (to p24)
• Workstations that connect to a switch are on a
switched segment
(to p21)
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Switches (continued)
• The backplane of a switch is fast enough to support
multiple data transfers at one time
• A switch that employs cut-through architecture is passing
on frame before entire frame has arrived at switch
• Multiple workstations connected to a switch use
dedicated segments
(to p25)
– This is a very efficient way to isolate heavy users from the
network
(to p26)
• Can allow simultaneous access to multiple servers, or
multiple simultaneous connections to a single server
– The use of switches in virtual LANs
– Full duplex Switches (to p33)
(to p31)
(to p3)
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Switches (continued)
Or
(to p23)
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FIGURE 12-19
A switch allows simultaneous connection of LAN segments.
(to p3)
(to p20)
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Why Segment or Internetwork
Local Area Networks?
• To separate / connect one corporate division
with another
• To connect two LANs with different protocols
• To connect LAN to Internet
• To break LAN into segments to relieve traffic
congestion
• To provide a security wall between two different
types of users
(to p20)
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Switches (continued)
(to p21)
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Isolating Traffic Patterns and
Providing Multiple Access
• Whether shared or dedicated segments are
involved, the primary goal of a switch is to
isolate a particular pattern of traffic from other
patterns of traffic or from the remainder of the
network
(to p27)
• Switches, because of their backplane, can also
allow multiple paths of communications to
(to p28)
simultaneously occur
• Alternative design
(to p29)
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Isolating Traffic Patterns and
Providing Multiple Access (continued)
(to p26)
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Isolating Traffic Patterns and
Providing Multiple Access (continued)
(to p26)
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Isolating Traffic Patterns and
Providing Multiple Access (continued)
• Using a pair of routers, it is possible to
interconnect to switched segments, essentially
(to p30)
creating one large local area network
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Isolating Traffic Patterns and
Providing Multiple Access (continued)
(to p21)
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Virtual LANs
• Virtual LAN (VLAN) – logical subgroup within a
LAN that is created via switches and software
rather than by manually moving wiring from one
network device to another
• Even though employees and their actual
computer workstations may be scattered
throughout the building, LAN switches and VLAN
software can be used to create a “network within
a network”
(to p32)
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Virtual LANs (continued)
• A relatively new standard, IEEE 802.1Q, was
designed to allow multiple devices to
intercommunicate and work together to create a
virtual LAN
• Instead of sending technician to a wiring closet
to move a workstation cable from one switch to
another, an 802.1Q-compliant switch can be
remotely configured by a network administrator
(to p21)
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Full-Duplex Switches
• Allow for simultaneous transmission and
reception of data to and from a workstation
• This full-duplex connection helps to eliminate
collisions
• To support a full-duplex connection to a switch,
(to p34)
at least two pairs of wires are necessary
– One for the receive operation
– One for the transmit operation
– Most people install four pairs today, so wiring is
not the problem
(to p21)
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Full Duplex Switches (continued)
(to p33)
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Routers
• Connects a LAN to a LAN, a LAN to a WAN or a
WAN to a WAN
• Accepts an outgoing packet, removes any LAN
headers and trailers, and encapsulates the
necessary WAN headers and trailers
(to p36)
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Routers (continued)
• Because a router has to make wide area network
routing decisions, router has to dig down into the
network layer of the packet to retrieve the network
destination address
– Thus, routers are often called “layer 3 devices”
– They operate at the third layer, or OSI network layer,
of the packet
• Routers often incorporate firewall functions
• An example of a router’s operation is shown on the
next slide
(to p37)
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Routers (continued)
(to p3)
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FIGURE 12-20
Bridges connect networks that use the same protocols. Gateways connect networks that use dissimilar protocols.
(to p3)
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FIGURE 12-21
LANs interconnected with a backbone LAN.
(to p3)
(FDDI)
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FIGURE 12-22
This diagram shows how a LAN switch can be used to connect several LANs together and to connect LANs to a WAN or MAN.
(to p3)
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LAN Internetworking In Action:
A Small Office Revisited
• Recall the In Action example from Chapter
Seven
– A small office with 20 workstations in one room
and 15 workstations in another room were
connected to a server via 100BaseTX
– One hub was kept in a closet near the 20
workstations while a second hub was near the
server
(to p42)
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LAN Internetworking In Action:
A Small Office Revisited (continued)
(to p43)
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LAN Internetworking In Action:
A Small Office Revisited (continued)
• Now Hannah wants to connect the LAN to the
Internet
– She adds a router next to the server and
connects it to the hub
– She connects the router to a high-speed
telephone line such as a T-1 service
– She will also have to program the router to
perform IP addressing and firewall functions
(to p44)
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LAN Internetworking In Action:
A Small Office Revisited (continued)
(to p45)
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LAN Internetworking In Action:
A Small Office Revisited (continued)
• Now network usage is so high that Hannah must
consider segmenting the network
– She decides to install a database server near the
original server and replace both hubs with
switches
(to p46)
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LAN Internetworking In Action:
A Small Office Revisited (continued)
(to p2)
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