Transcript Virtual LANs

Chapter 15
Connecting LANs,
Backbone Networks,
and Virtual LANs
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15-1 CONNECTING DEVICES
LANs do not normally operate in isolation they are
connected to one another or to the Internet
To connect LANs, or segments of LANs, we use
connecting devices
Topics discussed in this section:
1.Passive Hub
2.Repeater and Active hub
3.Bridges and Two-Layer Switches
4.Routers and Three-Layer Switches
5.Gateways
15.2
Figure 15.1: Five categories of connecting devices
connecting devices divided into five different categories
based on the layer in which they operate in a network.:
1.Below the physical layer: passive hub
2.At the physical layer: repeater or active hub
3.At the physical and data link layers: bridge or two-layer switch
4.At the physical, data link, network layers: router or three-layer
switch
5.At all five layers: gateway
15.3
Passive Hubs
15.4
Passive Hubs
Passive hub is just a connector.
 In a star-topology Ethernet LAN, it is just a point
where signals coming from different stations collide.
 The hub is the collision point.
 This type of hub is part of the media
 its location in the Internet model is below the
physical layer.
15.5
Repeaters
15.6
Repeaters
A repeater operates only in the physical layers
Can extend the physical length of a LAN
Receive the signal before it becomes too weak or
corrupted and regenerates the original bit pattern
Do not actually connect two LANs
connects two segments of the same LAN
segments connected are still part of one single
LAN
A repeater cannot connect two LANs of different
protocols
15.7
Repeaters
Example :
repeater can overcome 10Base5 Ethernet length
restriction
the length of the cable is limited to 500 m
divide the cable into (500 m) sections and connect
them with repeaters
The whole network is still considered one LAN
Portions of the network separated by repeaters are
called segments
Repeaters acts as two-port node
15.8
Repeaters
Repeaters is a regenerator, not an amplifier
15.9
Amplifier
Repeater
Cannot discriminate
between the intended
signal and noise.  It
amplifies equally
everything fed into it
regenerates the signal
 receives a
weakened or corrupted
signal, creates a copy,
bit for bit, at the
original strength
Function of a Repeater
Location of a repeater on a link is vital
it must placed on the line before the legibility of the
signal becomes lost can still read the signal well
enough to determine the intended voltages and
replicate them in their original form
15.10
Repeater
NOTE
•A repeater connects segments of a LAN.
•A repeater forwards every frame; it has no filtering
capability.
•A repeater is a regenerator, not an amplifier.
15.11
Active Hubs
Actually a multiport repeater
Used to create connections between stations in a
physical star topology
we have seen examples of hubs : 10Base-T
Can also be used to create tree topology to
removes the length limitation of 10Base -T (100 m)
17.12
Bridges
15.13
Bridges
Operates in both the physical and the data link layer
physical layer : regenerates the signal
data link layer : check the physical (MAC) addresses
(source and destination) contained in the frame
Bridge has filtering capability, but repeaters has not.
checks the MAC (physical) address of the destination
when receives a frame, and decide if the frame should
be forwarded or dropped
forwards the new copy only to the segment (specific
port) to which the address belongs
Bridge has a table that maps addresses to the port.
17.14
Bridges
Bridge has a table to:
Maps address to ports.
Used in filtering decisions.
15.15
Bridges: Transparent Bridges
Bridges in which the stations are completely unaware of
the bridge’s existence the stations does not
reconfigured when a bridge is added or deleted.
A system equipped with transparent bridges must meet
three criteria:
1.Frame must be forwarded correctly one station to another.
2.The forwarding table is automatically made by learning frame
movements in the network.
3.Loops in the system must be prevented.
Learning:
early bridges had static forwarding table
Administrated manually enter each table entry
simple, but not practical
better solution
dynamic table management that maps addresses to ports
automatically
bridge gradually learns from the frame movement
15.16
Bridges: Transparent
Bridges Destination physical address: used for the
forwarding decision (table lookup).
Source physical address: used for adding entries to the table
and for updating purposes.
1.A sends frame to D:flooding
2.E sends a frame to A: Forwarding
3.B sends a frame to C :flooding
17.17
Bridges: Transparent Bridges
Loop problem:
bridges are normally installed redundantly to make the
system more reliable
Two LANs may be connected by more than one bridge
they may create a loop  packet may be going round and
round
15.18
Bridges: Spanning Tree
Is graph in which there is no loop
Create a topology in which each LAN can be reached
from any other LAN through one path only (no loop)
Create a logical topology that overlays physical
topology which can not be changed
To find the spanning tree we need to Assign a cost
(metric) to each LAN
The interpretation of the cost is left up to network
admin
It may be the path with :
Minimum hops, (shortest distance)
Minimum delay, or maximum bandwidth
17.19
Spanning Tree with Minimum hops
The hop count is normally 1 from a bridge to the LAN
and 0 in the reverse direction.
15.20
Spanning Tree
Finding the spanning tree involves four steps:
1.Select root bridge : bridge which has the smallest built-in
ID
2.Mark one port of each bridge (except for the root bridge)
as the root port
Root port : port with the least-cost (minimum number of hops)
path from the bridge to the root bridge
if two ports have the same least-cost value, choose one
3.Choose a designated bridge for each LAN
The bridge with the least-cost path from the LAN to the root
bridge
The corresponding port is the designated port
4.Mark the root port and designated port as forwarding
port, others as blocking port
15.21
Example 1: Spanning Tree with Minimum hops
15.22
Example1: Spanning Tree with Minimum hops
R.P.
DP
R.P.
BP X
DP
R.P.
X
DP
DP
Note: B3 ID( has 3 ports) > B4 ID
X: blocked port
15.23
X BP
BP
R.P.
Spanning Tree with Minimum hops
There is only one single path from any LAN to any
other LAN
15.24
Bridges: Source Routing Bridges
Another way to prevent loops in a system is to use
source routing bridges
In source routing, filtering frames ( forwarding and
blocking) is performed by the source station
Sending station defines the bridges that the frame
must visit.
The addresses of these bridges are included in the
frame.
The frame contain the source and destination
address, and the address of all the bridges to be
visited
Used with Token Ring LANs( Not very common today)
15.25
Bridges Connecting Different LANs
Theoretically a bridge should be able to connect LANs using
different protocols at the data link layer
There are many issues to be considered:
Frame format
Each LAN type has its own frame format. Compare an Ethernet
frame with wireless LAN frame
Maximum data size
Needs fragmentation/reassembly
No protocol at the data link layer allows the
fragmentation/reassembly
fragmentation/reassembly is allowed in the network layer
Bridge discard any frames too large for its system
Data rate
each LAN type has its own data rate
Bridge must buffer the frame to compensate for this difference
15.26
Bridges: Bridges Connecting Different LANs
Bit order
Each LAN type has its own strategy in sending of
bits. Some send the MSB in a byte first; others the
LSB first
Security
Some LANs such as wireless, implement security
measurements in the data link layer. Other LANs,
such as Ethernet ,do not
Multimedia support
Some LANS support multimedia and the quality of
services needed for this type ; others do not.
15.27
Two-Layer Switch
15.28
Two-Layer Switch
Performs at the physical and data link layers.
Is a bridge with many ports (multi port bridge) Design
that allows better (faster) performance
No collision
Filtering based on the MAC address of the frame it
received (like bridge)
Builds switching table by “learning” MAC host addresses
from source addresses of incoming packets
Unknown destination addresses are flooded out other
ports
Broadcast frames are flooded out other ports.
New two-layer switches (called cut-through switches):
have been designed to forward the frame as soon as they
check the MAC addresses in the header of the frame( first
6-bytes).
15.29
Routers
15.30
Routers
Three-layer devices that routes packets based on their logical
addresses (IP)
Connects LANs and WANs in the Internet.
Has a routing table that is used for making decisions about the
route.
Routing table are dynamic and updated using routing protocol.
Builds routing table by neighbor routers using routing protocols
15.31
Three layer switch
Is a router, but a faster and more sophisticated.
•The switching fabric in a three-layer switch allows
faster table lookup and forwarding.
•We can use the terms router and three-layer switch
interchangeably.
15.32
Gateway
Normally a computer that operates in all five layers of the
Internet or seven layers of OSI model.
It takes an application message, reads it, and interrupts it
It used as connecting device between two internetworks
that use different models.
Can provide security( filter unwanted application-layer
messages)
15.33
Backbone Networks
Backbone network allows several LANs to be
connected
No station is directly connected to the backbone
It is itself a LAN that uses a LAN protocol such as
Ethernet
Discuss only the two most common :
Bus Backbone
Star Backbone
15.34
Bus Backbone
The topology of the backbone is a bus.
Backbone itself can use one of the protocols that support a
bus topology such as 10Base5 or 10Base2
normally used as a distribution backbone to connect different
buildings in an organization
example : one that connect buildings on a campus
15.35
Star Backbone
Topology is a star : sometimes called a collapsed or
switched backbone
It is just one switch that connects the LANs
Mostly used as a distribution backbone inside a multifloor
building
15.36
Connecting Remote LANs
Another common application for a backbone network
useful when a company has several offices with LANs and
needs to connect them
connection can be done through bridges, sometimes called
remote bridges
connect LANs and point-to-point networks (leased
telephone lines or ADSL lines)
point-to-point link can use a protocol such as PPP
15.37
NOTE
A point-to-point link acts as a LAN in a
remote backbone connected by remote
bridges
15.38
Virtual LANs
In a switched LAN, change the work group mean physical
changes in the network configuration.
What happens if we need
a virtual connection between
two stations belonging to two
different physical LANs?
 Virtual LANs
15.39
Virtual LANs
A network of stations that behave as if they are connected to the same
LAN even though they may actually be physically located on different
segments of a LAN
VLANs are configured through software rather than hardware, which
makes them extremely flexible
the whole idea of VLAN technology : divide a LAN into logical, instead
of physical, segments
a LAN can be divided into several logical LANs called VLANs
each VLAN is a workgroup in the organization.
One of the biggest advantages is that when a station moves from one
group to another, without any hardware reconfiguration.
15.40
Virtual LANs
One of the biggest advantages is that when a station moves
from one group to another, without any hardware
reconfiguration.
15.41
NOTE :
VLANs create broadcast domains
15.42
Virtual LANs
All members belonging to a VLAN can receive broadcast
messages sent to that particular VLAN
stations in a VLAN communicate with one another as though they
belonged to a physical segment
VLAN technology even allows the grouping of stations connected to
different switches in a VLAN
peoples in different buildings (LAN) could be in the same workgroup
15.43
Virtual LANs Membership
Membership is characterized by:
Port numbers,
MAC addresses,
IP addresses,
Multicast IP addresses
A combination of the above
15.44
Advantages of VLAN
1.Cost and time reduction:
Can reduce the migration cost of stations from one
group to another
physical reconfiguration takes time and is costly
it is much easier and quicker to move it using
software
2.Creating virtual workgroup
3.Security:
provide an extra measure of security : people belonging
to the same group can send broadcast messages with
the guaranteed assurance that users in other groups
will not receive these messages
15.45