Department of Engineering Science
ES465/CES 440, Intro. to Networking & Network Management
Repeaters, Bridges, & Switches
http://www.sonoma.edu/users/k/kujoory
References
• “Computer Networks & Internet,” Douglas Comer, 6th ed, Pearson, 2014, Ch 17,
Textbook, 5th ed, slides by Lami Kaya (LKaya@ieee.org) with some changes.
• “Computer Networks,” A. Tanenbaum, 5th ed., Prentice Hall, 2011, ISBN:
13:978013212695-3.
• “Computer & Communication Networks,” Nader F. Mir, 2nd ed, Prentice Hall, 2015, ISBN:
13: 9780133814743.
• “Data Communications Networking,” Behrouz A. Forouzan, 4th ed, Mc-Graw Hill, 2007
• “Data & Computer Communications,” W. Stallings, 7th ed., Prentice Hall, 2004.
• “Computer Networks: A Systems Approach," L. Peterson, B. Davie, 4th Ed., Morgan
Kaufmann 2007.
Ali Kujoory
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Topics Covered
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17.1 Introduction
17.2 Distance Limitation & LAN Design
17.3 Fiber Modem Extensions
17.4 Repeaters
17.5 Bridges & Bridging
17.6 Learning Bridges & Frame Filtering
17.7 Why Bridging Works Well
17.8 Distributed Spanning Tree
17.9 Switching & Layer 2 Switches
17.10 VLAN Switches
17.11 Bridging Used with Other Devices
Ali Kujoory
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17.1 Introduction
• This chapter
– discusses two important concepts:
• Mechanisms that can extend a LAN across a longer
distance &
• LAN switching
– introduces repeaters, hubs, bridges, & the spanning
tree algorithm used to prevent forwarding loops
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17.2 Distance Limitation & Design
• The major parameters an
engineer needs to consider
in the design a network are:
– Capacity
– Maximum cost
– Distance that can be
achieved at a given cost
• Limitation of distance arises
since hardware is
engineered to emit a fixed
amount of energy
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– At distance beyond the
design limit, stations will not
receive sufficiently strong
signal & errors occur
• Max length specification is
fundamental part of LAN
technology
• LAN hardware will not work
correctly over wires that
exceed the bound
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17.3 Fiber Modem Extensions
• Most extension mechanisms use
standard interface &
– insert additional hardware
components that can relay signals
across longer distances
• The simplest LAN extension
mechanism consists of an optical
fiber & a pair of fiber modems
– used to connect a computer to a
remote Ethernet
• Fig. 17.1 illustrates the
interconnection
• Each of the fiber modems contains
hardware to perform two functions:
– accept packets over the Ethernet
interface & send them over the
optical fiber &
– accept packets that arrive over the
optical fiber & send them over the
Ethernet interface
Fig. 17.1 Illustration of fiber modems used to provide
a connection between a computer & a remote Ethernet.
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17.4 Repeaters
• A repeater is an analog device
used to propagate LAN signals
over long distances
– A repeater does not understand
packets or signal coding, instead
– it merely amplifies the signal
received & transmits the
amplified version as output
• Repeaters were used
extensively with the original
Ethernet, & have been used
with other LAN technologies
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– Recently, repeaters have been
introduced with infrared receivers
• to permit a receiver to be located
at a longer distance from a
computer
– Consider a situation in which the
infrared receiver for a cable
television controller must be in a
different room than the controller
• Fig. 17.2 illustrates an infrared
sensor extended with a
repeater
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Figure 15.3 Function of a repeater
15.7
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17.5 Bridges & Bridging
• A bridge is a mechanism
that connects two LAN
segments
• The bridge listens in
promiscuous mode on
each segment, i.e.,
– receives all packets sent on
the segment
promiscuous = not restricted to one port
(go to all ports as in a Hub)
• When it receives a valid
frame from one segment
– the bridge forwards a copy of
the frame to the other
segment
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• Thus, two LAN segments
connected by a bridge appear
to behave like a single LAN
– a computer connected to
either segment can send a
frame to any computer on the
both segments
• A broadcast frame is delivered
to all computers
– Thus, computers do not know
whether they are connected
to a single LAN segment or a
bridged LAN
• Bridge technology is also
included in a cable & DSL
modem
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17.6 Learning Bridges & Frame Filtering
– the bridge must forward a
copy of each broadcast or
multicast frame
• Bridges do not blindly
forward a copy of each
frame from one LAN to
another, instead,
– A bridge uses MAC
addresses to perform filtering
• A bridge examines the
destination address in a
frame, &
– does not forward the frame
onto the other LAN segments
unless necessary
• If the LAN supports
broadcast or multicast
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• to make the bridged LAN
operate like a single LAN
• How can a bridge know
which computers are
attached to which
segments?
– Most bridges are called
adaptive or learning bridges
• because they learn the
locations of computers
automatically
– To do so, a bridge uses
source addresses
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17.6 Learning Bridges & Frame Filtering
• When a frame arrives from a
given segment
– the bridge extracts the source
address from the header, &
– adds the address to a list of
computers attached to the
segment
• Bridge uses the MAC
address to determine
whether to forward the frame
• A bridge learns that a
computer is present on a
segment as soon as the
computer transmits a frame
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• Consider the bridged LANs
in Fig. 17.3, &
• the example of a learning
bridge in Fig. 17.4, which
– lists a sequence of packet
transmissions,
– the location information that
the bridge has accumulated
at each step in time, &
– the disposition of the packet,
• i.e., the segments over which
the packet is sent
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Figure 15.5 A bridge connecting two LANs
15.11
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Figure 15.6 A learning bridge and the process of learning
15.12
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17.6 Learning Bridges & Frame Filtering
Fig 17.3 Illustration of
six computers connected
to a pair of bridged LAN
segments.
Does not know
yet where B is
Fig 17.4 Example of
a learning bridge with
computers A, B, & C
on one segment &
computers X, Y, & Z
on another.
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17.7 Why Bridging Works Well
• A bridged network can
exhibit higher overall
performance than a single
LAN
• A bridge permits
simultaneous transmission
on each segment
• In Fig. 17.3, e.g.,
– computer A can send a
packet to computer B
– at the same time computer X
sends a packet to computer Y
– Although it receives a copy of
each packet
• because each packet has
been sent to a destination on
the same segment as the
source
– the bridge merely discards
the two frames without
forwarding them
• A bridge permits
simultaneous activity on
attached segments
– a pair of computers on one
segment can communicate at
the same time as a pair of
computers on another
segment
• the bridge will not forward
either of them
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17.8 Distributed Spanning Tree (DST)
• Consider the four LAN
segments already connected by
three bridges & a fourth bridge
about to be inserted
• Assume the computers (not
shown) are plugged into each
hub as in Fig. 17.5
• The network operates well
before the 4th bridge inserted
– Computers can send unicast
frames to one another, also
– Broadcast & multicast work
well
• When the 4th bridge is inserted,
the loop becomes problematic
Fig. 17.5 Illustration of a bridged network
with a fourth bridge about to be inserted.
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Figure 15.7 Loop problem in a learning bridge
15.16
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17.8 Distributed Spanning Tree (DST)
• In Fig. 17.5 unless at least
one bridge is prevented from
forwarding broadcasts,
– copies of a broadcast frame
will go around the loop for
ever
– Computers attached to hubs
receive an endless # of
copies
• To prevent cycle of endless
loop, bridges implement an
algorithm that computes a
Distributed Spanning Tree
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• DST views bridges as nodes
in a graph & imposes a “tree
on a graph” (= without
loop)
• DST was originally
developed by DEC called
“Spanning Tree Protocol
(STP) in 1985
• STP consists of three steps:
1. Root election
2. Shortest path computation
3. Forwarding
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17.8 Distributed Spanning Tree (Spanning Tree Protocol)
1. Root election
– bridges multicast a packet
that contains their bridge ID,
&
o the bridge with the smallest ID
is chosen
– To permit a manager to
control the election, a bridge
ID consists of two parts:
o a 16-bit configurable priority #
o a 48-bit MAC address
2. Shortest path
computation
– Each bridge computes a
shortest path to the root
bridge
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-- Links included in the shortest
paths of all bridges form the
spanning tree
3. Forwarding
– An interface that connects to
the shortest path is enabled
for forwarding packets;
o an interface that does not lie
on the shortest path is
blocked
• In STP, Ethernet bridges
communicate amongst
themselves using a multicast
address that is reserved for
STP
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17.8 Distributed Spanning Tree (Spanning Tree Protocol)
• Variations of STP have been
designed & standardized
– IEEE created a standard
named 802.1d (in 1990)
– the standard was updated in
1998
• IEEE standard 802.1q
provides a way to run STP
on a set of logically
independent networks, that
– share a physical medium
without any confusion or
interference
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• Cisco created a proprietary
version of STP, Per-VLAN
Spanning Tree (PVST) for
use on a VLAN switch
• IEEE standard 802.1w
introduced the Rapid STP
(RSTP) has been
incorporated in 801.1d-2004
(in 1998), &
– now replaces STP, some
versions are
• Multiple Instance STP (MISTP)
• Multiple STP (MSTP)
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17.9 Switching & Layer 2 Switches
• An Ethernet switch,
sometimes called a Layer 2
switch is an electronic
device that resembles a hub
– a switch provides multiple
ports that each attach to a
single computer &
– a switch allows computers to
send frames to one another
• The difference between a
hub & a switch arises from
the way the devices operate:
– while a switch is a digital
device that forwards packets
– We can think of a hub as
simulating a shared
transmission medium
– We think of a switch as
simulating a bridged
network that has one
computer per LAN segment
• Fig. 17.6 illustrates the
conceptual use of bridges in
a switch
– a hub operates as an analog
device that forwards signals
among computers
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17.9 Switching & Layer 2 Switches
Ali Kujoory
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17.9 Switching & Layer 2 Switches
• A switch does not contain
separate bridges
– a switch consists of an
intelligent interface attached
to each port &
– a central fabric that provides
simultaneous transfers
• An interface contains
– a processor, memory, &
other hardware needed to
accept a packet
– consult a forwarding table &
– send the packet across the
fabric to the correct output
port
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• An interface can buffer
arriving packets when an
output port is busy
• Fig. 17.7 illustrates the
architecture
– Physically, switches are
available in many sizes (ports)
• Advantage of using a switched
LAN instead of a hub is
parallelism
– Although a hub can only support
one transmission at a time
• a switch permits multiple
transfers to occur at the same
time, provided the transfers are
independent
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17.9 Switching & Layer 2 Switches
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17.10 VLAN Switches
• Virtual Local Area Network (VLAN) switches
• The concept is straightforward:
– allow a manager to configure a single switch to
– emulate multiple, independent switches
• A manager can specify a set of ports on the switch &
designates them to be on virtual LAN 1
– designates another set of ports to be on virtual LAN 2, & so on
• When a computer on virtual LAN 2 broadcasts a packet
– only those computers on the same virtual LAN receive a copy
– i.e., once configured, a VLAN switch makes it appear that there are
multiple switches
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17.10 VLAN Switches
• Dividing computers into separate broadcast domains
does not appear important
– until one considers a large company or a service provider
• In each case, it may be important to guarantee that a set
of computers can communicate
– without others receiving the packets &
– without receiving packets from outsiders
• E.g., a company may choose to provide a firewall
between computers in the CEO's office & other
computers in the company
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17.11 Multiple Switches & Shared VLANs
• Switches are usually placed
in physical proximity to
computers
• E.g., an organization, may
choose to place a switch on
each floor
– Locating a switch near a set
of computers reduces # of
wires that must be run
– Conventional switches can be
interconnected for a large
single network
– Reduces the chances of
collisions on a large switch
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• Can VLAN operate across
multiple switches? YES
– Not without extra support
– IEEE802.1Q allows that with
additional VLAN tag, Fig.
17.8
• Without VLAN tag multiple
switches cannot manage
multiple VLANs effectively
– Imagine if both VLAN1,
VLAN2, & VLAN3 span
across SwitchA, SwitchB, &
SwitchC
• It will very time-consuming to
configure the switches with the
organizational changes
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17.11 Multiple Switches & Shared VLANs
• VLAN tags is used when computers attached to each switch in
a network belong to different departments
• The 4-byte VLAN tag is specified in IEEE 802.1Q
Fig. 17.8 The format of an Ethernet frame with an 802.1Q VLAN tag.
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17.11 Multiple Switches & Shared VLANs
Fig. 17.9 An example of how 802.1Q frame format is used
with interconnected VLAN switches
• 802.1Q tag allows multiple
VLAN switches to be
interconnected
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• When a PC broadcasts a frame
– Whereas the frames between
switches use 802.1Q format
– Frames between each switch &
the computers use the standard
format without tag
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PCs
– The corresponding switch
delivers a copy to each local port
that is a part of VLAN &
• Inserts the VLAN tag, & sends the
frame across the inter-switch link.
– The second switch receives the
frame, removes the tag &
• Delivers a copy to PCs of that
VLAN
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17.12 The Importance of Bridging
• Although bridges are not
sold as stand-alone devices,
bridging is a fundamental
concept that has been
incorporated into many
devices
• E.g., a DSL or cable modem
provides a form of bridging:
• Some wireless technologies
also use a form of bridging
to transfer frames from a
mobile device to a provider's
network
– provides an Ethernet
connection at a subscriber's
residence &
– transfers Ethernet packets
between the subscriber's
location & the provider's
network
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