5/8/2010

Chapter 15
Connecting LANs,
Backbone Networks,
and Virtual LANs

Dr. Mznah Al-Rodhaan

Based on
15.1 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

15--1 CONNECTING DEVICES

15

In this section, we divide connecting devices into five

different categories based on the layer in which they
operate in a network
network..

Topics discussed in this section:

Passive Hubs
Active Hubs
B id
Bridges
Two-Layer Switches
Routers
Three-Layer Switches
Gateways
Based on
15.2 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

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Five categories of connecting devices

Below the physical layer: Passive hub

Based on
15.3 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

Five categories of connecting devices

At the physical layer:
• Repeater
p
• Active hub

Based on
15.4 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

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Five categories of connecting devices

At the physical and data link layers:
• Bridge
• two-layer switch

Based on
15.5 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

Five categories of connecting devices

At the physical, data link, network layers:
• Router
• Three-layer switch

Based on
15.6 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

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Five categories of connecting devices

At all five layers: Gateway

Based on
15.7 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

A repeater connecting two segments of a LAN

A repeater forwards every frame;

it has no filtering capability.
Based on
15.8 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

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Function of a repeater

A repeater is a regenerator, not an amplifier.

Location of a repeater on a link is vital

Based on
15.9 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

A hierarchy of hubs

A hub is a multiport repeater

used to create connections between stations in a
physical star topology.

Can also be used to create multiple levels of hierarchy

removes the length limitation of 10Base-T (100 m)

Based on
15.10 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

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A bridge connecting two LANs

Bridge has filtering capability unlike repeaters.
‰ Checks the (physical) address of the destination when receives a
frame, forwards the new copy only to the segment (specific port)
to which the address belongsg
‰ has a table that maps address to ports.
A bridge has a table used in filtering decisions.
A bridge does not change the physical (MAC) addresses in a frame.

Based on
15.11 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

Transparent Bridges

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:
‰ Frame must be forwarded; one station to another.
‰ The forwarding table is automatically made by learning
frame movements in the network.
‰ Loops in
i the
h system must beb prevented.
d

Forwarding : must correctly forward the frames

Based on
15.12 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

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Transparent Bridges

Learning:
early bridges had static forwarding table
‰ manually administrated

‰ simple,
simple but not practical
better solution
‰ dynamic table management that maps addresses to
ports automatically
‰ bridge gradually learns from the frame movement

Destination address is used for the forwarding decision (table

lookup).

Source address is used for adding entries to the table and for
updating purposes.

Based on
15.13 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

A learning bridge and the process of learning

Based on
15.14 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

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Loop problem in a learning bridge

Loop problem:
bridges are normally installed redundantly to make the system more
reliable; if a two LANs are connected by more than one bridge
they may create a loop.

Based on
15.15 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

Spanning Tree

A graph in which there is no loop

Create a topology in which each LAN can be reached from any other LAN
through
g one path
p onlyy ((no loop)
p)
Create a logical topology that overlays physical topology which can not be
changed

To find the spanning tree

Assign a cost (metric) to each arc according to:
• Minimum hops,
• Minimum delay, or
• minimum bandwidth
Mi i
Minimum hops
h
The hop count is normally 1 from a bridge to the LAN and 0 in the reverse
direction

Based on
15.16 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

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Figure 15.8 A system of connected LANs and its graph representation

Based on
15.17 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

Figure 15.9 Finding the shortest paths and the spanning

tree in a system of bridges

Based on
15.18 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

9
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Figure 15.10 Forwarding and blocking ports after using spanning

tree algorithm

Based on
15.19 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

Root bridge in spanning tree algorithm

Source of this example: http://en.wikipedia.org/wiki/Spanning_tree_protocol

Based on
15.20 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

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Root and designated ports after using spanning tree algorithm

Based on
15.21 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

Blocked ports after using spanning tree algorithm

Based on
15.22 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

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Source Routing Bridges

„ Another way to prevent loops in a system.
„ Sending station defines the bridges that the frame

must visit.
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)

Based on
15.23 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

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

„ No protocol at the data link layer allows the

fragmentation/reassembly; this is allowed in the network

layer
„ Bridge discard any frames too large for its system

„ D t rate
Data t
„ each LAN type has its own data rate

„ Bridge must buffer the frame to compensate for this

difference
„ bit order, security, multimedia support (quality of
service), ….
Based on
15.24 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

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Two-Layer Switch
„ Performs at the physical and data link layers.
„ It is a bridge with many ports
„ Design that allows better (faster) performance
„ No collision
„ Filtering based on the MAC address of the frame it
received (like bridge)
„ More sophisticated( buffer, switching factory that
forward frames faster)
„ New two-layer switches (called cut-through switches)
‰ They have been designed to forward the frame as soon as
they check the MAC addresses in the header of the frame.
Based on
15.25 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

Routers
„ Three-layer devices that routes packets based on their
logical addresses (host-to-host addressing)
„ 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
„ No collision

Based on
15.26 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

13
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Three-layer Switch

„ Is a router, but a faster and more sophisticated.

„ The switching fabric in a three-layer
three layer switch allows
faster table lookup and forwarding.

Based on
15.27 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

Broadcast and Collision Domains

Switches/Bridges/Routers segment collision domain.

Device Collision domain

Repeater 1
Hub 1
Bridge number of ports
Switch number of ports
router number of ports

Stations may have dedicated bandwidth.

Based on
15.28 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

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Gateway
„ Normally operates in all five layers of the Internet or
seven layers of OSI model.
„ It takes an application message,
message reads it,
it and interrupts
it.
„ It used as connecting device between two
internetworks that use different models.(OSI or
Internet)
„ Can p provide security(
y( filter unwanted application-layer
pp y
messages)

Based on
15.29 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

15--2 BACKBONE NETWORKS

15

A backbone network allows several LANs to be

connected.. In a backbone network,
connected network no station is
directly connected to the backbone;
backbone; the stations are
part of a LAN, and the backbone connects the LANs
LANs..

Topics discussed in this section:

Bus Backbone
Star Backbone
Connecting Remote LANs

Based on
15.30 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

15
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Bus backbone

In a 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

Based on
15.31 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

Bus backbone

Based on
15.32 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

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Star backbone
In a star backbone, the topology of the backbone is
a star; the backbone is just one switch.
sometimes called a collapsed or switched backbone.
Mostly used as a distribution
backbone inside
a multi-floor building

Based on
15.33 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

Connecting remote LANs with bridges

connection can be done through bridges, sometimes called remote
bridges
„ connect LANs and point-to-point networks using leased
t l h
telephone lines
li or ADSL lines.
li
„ point-to-point link can use a protocol such as PPP.

A point-to-point link
acts as a LAN in a
remote backbone
connected by remote
bridges

Based on
15.34 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

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15--3 VIRTUAL LANs

15

We can roughly define a virtual local area network

(VLAN) as a local area network configured by
software, not by physical wiring.
wiring.

Topics discussed in this section:

Membership
Communication between switches
IEEE Standard
Advantages

Based on
15.35 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

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 extremely flexible

‰ One of the biggest advantages is that when a station is

physically moved to another location, it can stay on the same

VLAN without any hardware reconfiguration
„ the whole idea of VLAN technology : divide a LAN into logical
i t d off physical
instead h i l segments t
‰ a LAN can be divided into several logical LANs called

VLANs
‰ each VLAN is a workgroup in the organization

Based on
15.36 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

18
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Figure 15.15 A switch connecting three LANs

Based on
15.37 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

Figure 15.16 A switch using VLAN software

Based on
15.38 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

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VLAN characteristic
„ Any station can be logically moved to another VLAN
„ All members belonging to a VLAN can receive broadcast messages
sent to that particular VLAN
„ peoples in different buildings (LAN) could be in the same
workgroup
„ it groups stations belonging to one or more physical LANs into
broadcast domains
„ stations in a VLAN communicate with one another as though they

belonged to a physical segment.

VLANs create broadcast domains.

Based on
15.39 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

Figure 15.17 Two switches in a backbone using VLAN software

VLAN technology even allows the grouping of stations connected

to different switches in a VLAN

Based on
15.40 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

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15.3 Virtual LANs

Membership

‰ Membership
p is characterized byy port
p numbers,, MAC
addresses, IP addresses, Multicast IP addresses, or a
combination of the above

Configuration

‰ VLAN can be configured in one of three ways: manual,

semiautomatic and automatic
semiautomatic,

Based on
15.41 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

15.3 Virtual LANs

Advantages of VLAN
„ 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

„ can be use to create virtual workgroups

„ 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

Based on
15.42 th
Data Communications and Networking, 4 Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007 Dr. Mznah Al-Rodhaan

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