Local Area Networks - Internetworking

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• Chapter 8
• Local Area Networks - Internetworking

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Chapter Eight - Local Area Networks
Internetworking

Introduction Many times it is necessary to
connect a local area network to another local
area network or to a wide area network. Local
area network to local area network connections
are often performed with a bridge-like
device. Local area network to wide area network
connections are usually performed with a
router. A third device, the switch, can be used
to interconnect segments of a local area network.
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Why Interconnect? To separate / connect one
corporate division with another. To connect two
LANs with different protocols. To connect a LAN
to the Internet. To break a LAN into segments to
relieve traffic congestion. To provide a security
wall between two different types of users.
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Repeaters
Repeaters extendthe maximumallowed
distancebetween nodes.
Repeater
telnet

• Repeaters
• Repeaters repeat a signal from one port to
  another. A repeater forwards every frame
• Repeaters pass all traffic through without error
  checking or filtering..
• Repeaters pass collisions, too.

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Hubs As seen earlier, a hub interconnects two or
more workstations into a local area network. When
a workstation transmits to a hub, the hub
immediately resends the data frame out all
connecting links. A hub can be managed or
unmanaged. A managed hub possesses enough
processing power that it can be managed from a
remote location.
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Hubs

• very simple devices that pass all traffic in both
  directions between the LAN sections they link.
• same or different cable types
• pass on every message
• used to connect LANs of similar technology, or to
  extend the distance of one LAN
• Very similar to repeaters
• Also operates at the physical layer
• use physical layer protocols

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Hubs
Hub
Hubs make itvery easy to addand remove hostson
a network.
telnet

• Hubs
• Hubs propagate a signal received on one port to
  all other ports.
• Hubs propagate errors and collisions across
  ports, too.
• Hubs simplify the addition and removal of nodes
  on a LAN.
• Hubs are also used to connect network segments
  cabled with different media types.

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Can also create multiple level of hierarchy
15.8
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Hubs

• Advantages -
• inexpensive
• easy to Install
• can connect different media
• very little delay
• Disadvantages -
• limited distance between devices
• no protocol
• no error detection
• does not filter

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Bridges A bridge (or bridge-like device) can be
used to connect two similar LANs, such as two
CSMA/CD LANs. A bridge can also be used to
connect two closely similar LANs, such as a
CSMA/CD LAN and a token ring LAN. The bridge
examines the destination address in a frame and
either forwards this frame onto the next LAN or
does not. The bridge examines the source address
in a frame and places this address in a routing
table, to be used for future routing decisions.
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Bridges

• Connects two or more LANs at the link layer
• Extracts destination address from the frame.
• Looks up the destination in a table.
• Forwards the frame to the appropriate LAN
  segment.
• same or different cable types.
• forward only those messages that need to go out
  (filtering).
• learn whether to forward packets.
• Each segment can carry its own traffic

host
host
host
host
host
host
Bridge
host
host
host
host
host
host
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Bridges
Bridge
Bridges make it possible to segmentyour network
into separate collisiondomains to minimize
collisions and improve performance.
Hub
Hub
telnet
telnet
Separate Collision Domains

• Bridges
• Bridges provide all the functionality of a
  hub....
• Bridges filter frames by destination MAC, and
  segment a LAN into multiple collision domains.
• Bridges filter signal and timing errors.
• Bridges can be used to connect segments operating
  at different speeds.

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A bridge has a table used in filtering
decisions.

• It can check the destination address of a frame
  and decide if the frame should be forwarded or
  dropped.
• If the frame is to be forwarded, the decision
  must specify to port.
• A bridge has a table that maps address to
  ports.

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A bridge connecting two LANs
the bridge can check the physical/MAC addresses
contained in the frame.
15.15
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Transparent Bridges A transparent bridge does not
need programming but observes all traffic and
builds routing tables from this observation. This
observation is called backward learning. Each
bridge has two connections (ports) and there is a
routing table associated with each port. A bridge
observes each frame that arrives at a port,
extracts the source address from the frame, and
places that address in the ports routing
table. A transparent bridge is found with CSMA/CD
LANs.
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Transparent Bridges A transparent bridge can also
convert one frame format to another, but 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. The bridge removes
the headers and trailers from one frame format
and inserts (encapsulates) the headers and
trailers for the second frame format.
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A learning bridge and the process of learning
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Remote Bridges A remote bridge is capable of
passing a data frame from one local area network
to another when the two LANs are separated by a
long distance and there is a wide area network
connecting the two LANs. A remote bridge takes
the frame before it leaves the first LAN and
encapsulates the WAN headers and trailers. When
the packet arrives at the destination remote
bridge, that bridge removes the WAN headers and
trailers leaving the original frame.
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Spanning Tree Algorithm What happens if you have
many LANs interconnected with multiple bridges,
such as 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 How do we stop this from
happening? Disconnect one of the bridges? Maybe
we want bridge redundancy in case one bridge
fails. How about applying the spanning tree
algorithm. How is the algorithm applied?
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Spanning Tree Algorithm Step 1 Designate a Root
Bridge Step 2 Mark one port of each bridge as
the Root Port. The root port is the port with the
least-cost path from that bridge to the root
bridge. The root ports are denoted with an
asterisk in Figure 8-7b. Step 3 The next step
is to select a designated bridge for each LAN. A
designated bridge has the least-cost path between
that LAN and the root bridge. Mark the
corresponding port that connects that LAN to its
designated bridge with two asterisks (Figure
8-7b)
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Spanning Tree Algorithm Step 4 If a port has no
asterisks, that port is redundant and can be
removed. Keep all ports with one or two
asterisks. The resulting configuration is shown
in Figure 8-7c. Note there is now only one way to
get to any LAN or bridge from any other LAN or
bridge.
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Example Extended LAN with LOOPS
A
B
B9
B7
B5
F
C
D
K
B2
B1
J
E
H
G
B4
B
I
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Defining a Spanning Tree

• Basic Rules
• Bridge with the lowest ID is the root
• For a given bridge
• A port in the direction of the root bridge is the
  root port
• For a given LAN
• The bridge closest to the root (or the bridge
  with the lowest ID to break ties) is the
  designated bridge for a LAN
• The corresponding port is the designated port
• Bridges with no designated ports and ports that
  are neither a root port nor a designated port are
  not part of the tree.

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Spanning Tree Algorithm
A
B
Root
B9
B7
B5
D designated port
F
C
D
K
B2
B1
B1
R root port
J
E
H
G
B4
B
I
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Using a Spanning Tree Forwarding

• Forwarding
• Each bridge forwards frames over each LAN for
  which it is the designated bridge or connected by
  a root port

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Switches A switch is a combination of a hub and a
bridge. It can interconnect two or more
workstations, but like a bridge, it observes
traffic flow and learns. When a frame arrives at
a switch, the switch examines the destination
address and forwards the frame out the one
necessary connection. Workstations that connect
to a hub are on a shared segment. Workstations
that connect to a switch are on a switched
segment.
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Switches 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 the frame before the entire frame has
arrived at the switch. Multiple workstations
connected to a switch use dedicated segments.
This is a very efficient way to isolate heavy
users from the network. A switch can allow
simultaneous access to multiple servers, or
multiple simultaneous connections to a single
server.
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Switches
Switch
Switches are similar to bridges, but offer
multiple parallel communication channels across
ports for improved performance.
telnet
telnet

• Switches
• Switches provide all the functionality of a
  bridge ....
• Switches typically offer more ports than bridges.
  
• Switches allow for multiple, parallel channels of
  communication between ports.

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Virtual LANs A virtual LAN, or VLAN, is a logical
subgroup within a local area network that is
created via switches and software rather than by
manually moving wiring from one network device to
another Even though the 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.
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Virtual LANs 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 a 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
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Switches

• Cut-through switches
• use circuit-switching to immediately connect the
  port with the incoming message to the correct
  outgoing port
• very fast as decisions are done in hardware
• outgoing packet is lost if port is in use
• Store-and-forward switches
• copy the incoming packet to memory prior to
  processing the destination address -- transmit it
  when the outgoing port is ready

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Isolating Traffic Patterns with Switches 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
Switches, because of their backplane, can also
allow multiple paths of communications to
simultaneously occur
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Isolating Traffic Patterns with Switches Using a
pair of routers, it is possible to interconnect
to switched segments, essentially creating one
large local area network
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Full Duplex Switches A full duplex switch allows
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, at
least two pairs of wires are necessary - one for
the receive operation and one for the transmit
operation. Most people install four pairs today,
so wiring is not the problem.
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Network Servers Network servers provide the
storage necessary for LAN software. They are
usually the focal point for the network operating
system. Increasingly, network servers are
functioning as bridges, switches, and routers.
By adding the appropriate card, a server can
assume multiple functions.
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Routers The device that connects a LAN to a WAN
or a WAN to a WAN. A router accepts an outgoing
packet, removes any LAN headers and trailers, and
encapsulates the necessary WAN headers and
trailers. Because a router has to make wide area
network routing decisions, the router has to dig
down into the network layer of the packet to
retrieve the network destination address.
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Routers 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
routers operation is shown on the next slide.
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Routers

• connect two or more LANs that use the same or
  different data link protocols, but the same
  network protocol.
• same or different cable types
• operate at the network layer
• forward only messages that need to go out
• routers use the internetwork address
• internal routing tables
• only processes messages addressed to it

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Routers

• Advantages
• can mix-in-match protocols and convert them
• enable all ports to work at the same time
• can be used as an extra layer of security
• configurable
• high speed
• Disadvantages
• hard to configure and manage
• access lists must be kept current
• high maintenance/high training costs
• very expensive

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Switch vs Router vs Hub vs Bridge Vs Repeater Vs
Wireless Access Point

• Comparison and properties of a Hub
•  Hub is the simplest of these devices out of the
  five compared.
• Hubs cannot filter data so data packets are sent
  to all connected devices/computers. The device
  has to make decision if it needs the packet.
  This can slow down the network overall.
• Hubs do not have intelligence to find out best
  path for data packets. This leads to
  inefficiencies and wastage.
• Pretty much repeat signal on one end to another.
• Hubs are used on small networks where data
  transmission is not very high.

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• Comparison and properties of a Bridge
• A bridge is more complex than hub.
• A bridge maintains a MAC address table for both
  LAN segments it is connected to.
• Bridge has a single incoming and outgoing port.
• Bridge filters traffic on the LAN by looking at
  the MAC address.
• Bridge looks at the destination of the packet
  before forwarding unlike a hub.It restricts
  transmission on other LAN segment if destination
  is not found.
• Bridges are used to separate parts of a network
  that do not need to communicate regularly, but
  need to be connected.

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• Comparison and properties of a Switch
• A switch when compared to bridge has multiple
  ports.
• Switches can perform error checking before
  forwarding data.
• Switches are very efficient by not forwarding
  packets that error-ed out or forwarding good
  packets selectively to correct devices only.
• Switches can support both layer 2 (based on MAC
  Address) and layer 3 (Based on IP
  address) depending on the type of switch.
• Usually large networks use switches instead of
  hubs to connect computers within the same subnet.

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• Comparison and properties of a Router
• A router, like a switch forwards packets based on
  address.
• A router uses the IP address to forward packets.
  This allows the network to go across different
  protocols.
• Routers forward packets based on software while a
  switch (Layer 3 for example) forwards using
  hardware called ASIC (Application Specific
  Integrated Circuits)
• Routers support different WAN technologies but
  switches do not.
• Wireless Routers have Access Point built in.
• The most common home use for routers is to share
  a broadband internet connection. The router has a
  public IP address and that address is shared with
  the network. When data comes through the router
  it is forwarded to the correct computer.

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• Comparison and properties of a wireless access
  point
• Wireless Access Point bridges wireless and wired
  traffic.
• Wireless Access Point allows devices/computers to
  connect to LAN in a wireless fashion.
• Wireless Access Point allows wired and wireless
  devices work to communicate with each other.
• Comparison and properties of a Repeater
• Repeaters are built into the hubs or switches.
  Repeaters clean, amplify and resend the
  signals that have been weakened due to long
  cables traveling large distances.

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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.
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LAN Internetworking In Action A Small Office
Revisited 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.
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LAN Internetworking In Action A Small Office
Revisited 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.
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