Transcript File - JNNCE ECE Manjunath

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25\10\2010
Unit – 5
Connecting Devices
Backbone Networks
Virtual LANs
Unit-V Connecting LANs
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CONNECTING DEVICES
In this section, connecting devices are divided into five
different categories based on the layer in which they
operate in a network.
Passive Hubs
Active Hubs
Bridges
Two-Layer Switches
Routers
Three-Layer Switches
Gateways
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Connecting Devices
Five categories of connecting devices based on the layer in which they operate
in a network
1.
Those which operate below the physical layer (passive hub)
2.
Those which operate at the physical layer (repeater or an active hub)
3.
Those which operate at the physical & data link layer (bridge)
4.
Those which operate at he physical & data link &network layer (router
or 3 layer switch)
5.
Those which operate at all the five layer (a gateway)
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Passive Hub
is a connector
It connects the wires coming from different branches.
same or different cable types
pass on every message
used to connect LANs of similar technology, or to extend the
distance of one LAN
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Repeater
is a device that operates only in physical layer
It does not connects two LANs, it connects segments of a
LAN.
It is used to extend the physical length of a LAN
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Repeaters
forwards every frame; it has no filtering capability.
It does not amplify the signal it regenerates the signal
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Active Hubs
are a multiport repeater
It is usually used to create connections between stations in a
physical star topology
limited distance between devices
no protocol or rate conversion
no error detection
does not filter
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Active Hubs
HUB Devices
Repeater/Amplifier
HUB
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Active Hubs
A hierarchy of hubs
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Bridges
Operates in both the physical and the data link layer.
The difference between bridge and repeater is that, it has
filtering capability.
Can physical (MAC) addresses contained in the frame.
connect two LAN segments that use the same data link and
network protocol
same or different cable types
forward only those messages that need to go out (filtering)
“learn” whether to forward packets
internal routing table
combination of “black box” hardware and software
A bridge has a table used in filtering decisions.
A bridge does not change the physical (MAC) addresses in
a frame.
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Bridges
A bridge connecting two LANs
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Bridges
There are three types of bridges:
• Simple bridge
• Learning bridge
• Multiport bridge
Transparent Bridge
• A Transparent Bridge is bridge in which the stations are
completely unaware of the bridge’s existence
Transparent bridges must meet three criteria:
1. Frames must be forwarded from one station to another
2. Forwarding table must be automatically updated
3. Loops in the system must be prevented.
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Bridges
Learning:
• Bridges have forwarding tables.
• System administrator manually enter each table during
bridge setup
• If a station is added, or deleted or MAC address changed
which is not a rare event (Inserting a new network card).
• A solution to this is to map address to ports automatically.
• In this case table is dynamic in which bridge gradually
updates addresses to ports automatically.
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Bridges
A learning bridge and the process of learning
A sends frame to D, it flood in all ports but it notices A is in port 1
When E sends a frame to A it forwards only on port 1 and notices E is in port 3
B sends a frame to C it floods in all ports and notices B is in port 1
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Bridges
Loop problem
Networks usually have redundant bridges to make system
more reliable
If bridge fails another bridge takes over until the failed one is
repaired or replaced
Redundancy can create loops in the system, which is very
undesirable.
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Bridges
Loop problem in a learning bridge
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Bridges
Spanning Tree
In a graph theory, a spanning tree is graph in which there is
no loop but all the elements of the graph are connected.
In this case, creating bridged network in which LAN can be
reached from any other LAN through one path only.
Network consists of 4 LANS and 5 bridges
To find a spanning tree a cost (metric) is used.
Cost may be minimum hops, minimum path delay or path
with maximum bandwidth.
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Bridges
• The bridge with smallest ID is selected as root node.
• Assuming B1 has the smallest ID
• Bridge to LAN COST is 1 and LAN to Bridge cost is 0
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Bridges
Finding the shortest paths and the spanning tree in a system of bridges
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Bridges
Forwarding and blocking ports after using spanning tree
algorithm
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Switches
Connect more than two LAN segments that use the same
data link and network protocol.
Switches
• Two Layer switch
• Three Layer switch
Two Layer switch
• Performs at physical and data link layer
• Ex Bridge, can connect a few LANs together
• Makes filtering decision based on MAC address
Three Layer switch
• Performs at Network layer
• Ex, Routers
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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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Switches
Switches Interconnecting
Wing A
Wing B
First Floor Switch
Wing C
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Wing C
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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 internetwork address (IP or Logical address)
Internal routing tables
Only processes messages addressed to it
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Routers
Routers Interconnecting
Router
\
X.25 Network
Ethernet
the “cloud”
LAN2
Token Ring
LAN1
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Routers
Routers connecting independent LANs and WANs
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Gateways
is normally a computer that operates in all five layers of the
Internet or seven layers of OSI model
complex machines that are interfaces between two or more
dissimilar networks
Connecting device between tow internetworks that use
different models (one with OSI and other with Internet model)
connect two or more LANs that use the same or different
data link layer, network layer, and cable types
forwards only those messages that need to go out
a combination of both hardware and software
translates one network protocol to another
translates data formats
translates open sessions between application programs
Gateways can provide security
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BACKBONE NETWORKS
A backbone network allows several LANs to be
connected. In a backbone network, no station is directly
connected to the backbone; the stations are part of a
LAN, and the backbone connects the LANs.
Bus Backbone
Star Backbone
Connecting Remote LANs
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Bus Backbone
In a bus backbone, the topology of the backbone is a bus.
Normally used as a distribution backbone to connect
different buildings in an organization.
Each building may have single LAN or another
backbone(star).
Ex: single or multiple floor buildings on a campus, each
single floor usually has a single LAN, each multiple floor
building has a backbone (star) that connect each LAN. A bus
backbone can interconnect these LANs and backbone.
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Bus Backbone
Frame within a LAN will not be allowed go out by bridge.
If a station needs to send frame to station in another LAN the
bridge passes the frame to the backbone which is received
by the appropriate bridge and is delivered to the destination
LAN
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Star Backbone
In a star backbone, the topology of the backbone is a star;
the backbone is just one switch.
Star backbone are mostly used as backbone inside a
building.
In a multifloor building, there is one LAN for each floor, and
star backbone connects these LANs.
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Connecting remote LANs
A point-to-point link acts as a LAN in a remote backbone
connected by remote bridges.
Company has several offices need to be connected.
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VIRTUAL LANs
Virtual local area network (VLAN) is defined as a local
area network configured by software, not by physical
wiring.
Membership
Configuration
Communication between Switches
IEEE Standard
Advantages
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Connecting remote LANs
A switch connecting three LANs
10 stations grouped into 3 LANs,
If two engineers need to move from grop1 to group3 , rewiring is needed
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Connecting remote LANs
A switch using VLAN software
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Connecting remote LANs
Two switches in a backbone using VLAN software
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Membership
Characteristic to group a stations in a VLAN
Port Numbers:
• Switch port numbers. Ex stations connecting to ports
1,2,3, 7 to VLAN1 and stations connecting to ports 4,10,
and 12 belong to VLAN2
MAC addresses:
• 48 bit MAC address
IP addresses: 32 bit IP address
Combination:
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Configuration
How are the stations are grouped into different VLANs
Manual configuration:
• Network administrator uses VLAN software to manually
assign the stations into different VLANs at setup, based
on characteristic. Manually means administrator types the
port numbers, IP address …
Automatic configuration:
• Administrator groups stations based on the project
working.
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