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)
2
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)
3
FIGURE 12-18
Network interconnection can occur at various layers of the OSI model.
(to p3)
hub
4
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)
5
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)
6
Bridges (continued)
(to p6)
or
(to p8)
7
FIGURE 12-21
LANs interconnected with a backbone LAN.
(to p3)
(FDDI)
(to p6)
8
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)
9
Transparent Bridge (continued)
(to p9)
10
Transparent Bridge (continued)
(to p9)
11
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)
12
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!
13
Spanning Tree Algorithm (continued)
(to p15)
Data confusing
areas
14
Spanning Tree Algorithm (continued)
(to p16)
Solution method
15
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)
16
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)
17
Spanning Tree Algorithm (continued)
(to p17)
(to p17)
(to p17)
18
Remote Bridge (continued)
(to p12)
19
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)
20
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)
21
Switches (continued)
Or
(to p23)
22
FIGURE 12-19
A switch allows simultaneous connection of LAN segments.
(to p3)
(to p20)
23
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)
24
Switches (continued)
(to p21)
25
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)
26
Isolating Traffic Patterns and
Providing Multiple Access (continued)
(to p26)
27
Isolating Traffic Patterns and
Providing Multiple Access (continued)
(to p26)
28
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
29
Isolating Traffic Patterns and
Providing Multiple Access (continued)
(to p21)
30
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)
31
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)
32
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)
33
Full Duplex Switches (continued)
(to p33)
34
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)
35
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)
36
Routers (continued)
(to p3)
37
FIGURE 12-20
Bridges connect networks that use the same protocols. Gateways connect networks that use dissimilar protocols.
(to p3)
38
FIGURE 12-21
LANs interconnected with a backbone LAN.
(to p3)
(FDDI)
39
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)
40
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)
41
LAN Internetworking In Action:
A Small Office Revisited (continued)
(to p43)
42
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)
43
LAN Internetworking In Action:
A Small Office Revisited (continued)
(to p45)
44
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)
45
LAN Internetworking In Action:
A Small Office Revisited (continued)
(to p2)
46