Chapter 3, Network Connections
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Chapter Overview
A.
B.
C.
Bridging
Switching
Routing
Chapter 3, Lesson 1
Bridging
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1.
What Is a Bridge?
A.
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2.
Connecting LANs with a Bridge
A.
B.
C.
D.
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A data-link layer device that
1.
Connects networks
2.
Filters packets
a.
Propagates only packets destined for the other side of the bridge
b.
Can reduce network traffic and collisions
c.
Can lessen delays caused by high traffic levels
E.
Usually a hardware device with two ports
Connects two LANs or splits an existing LAN in half
Bridges operate in promiscuous mode, reading and processing all of
the packets transmitted over the network segments.
1.
Network interface adapters normally process only the packets that are
addressed to them and discard the rest.
Bridge functions
1.
Bridges can read the source and destination addresses in data-link layer
protocol headers.
a.
If the destination address of a packet arriving from one network
segment is that of a computer on the other segment, the bridge
transmits it out through the other port.
b.
If the destination address is that of a computer on the same network
segment as the computer that generated it, the bridge discards the
packet.
2.
The bridge eliminates unnecessary traffic from both segments.
3.
Bridges cannot operate any higher than the data-link layer and cannot
read any part of the packet other than the data-link layer protocol header.
Bridges and collisions
1.
A collision domain is a network (or part of a network) that is constructed
so that a collision occurs when two computers transmit packets at
precisely the same time.
2.
Adding a hub to a LAN increases its size but maintains a single collision
domain because hubs relay signals immediately at the physical layer,
without filtering them.
a.
F.
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G.
H.
2
Hubs do not relay packets immediately; they wait until the entire
packet is received.
3.
Because the bridge delays propagation, computers on opposite sides of
the bridge transmitting at the same time do not cause a collision.
4.
Adding a bridge to a LAN splits it into two collision domains, resulting
in fewer collisions and increased efficiency.
Bridges and broadcasts
1.
Terms
a.
A broadcast message is a packet transmitted to all of the computers
on the network.
b.
A unicast message is a packet transmitted to one computer.
c.
A multicast message is a packet transmitted to a group of computers.
d.
A broadcast domain is a group of computers that all receive broadcast
transmissions sent by any computer in the group.
2.
Broadcasts are essential to LANs.
a.
Used by Address Resolution Protocol (ARP), Dynamic Host
Configuration Protocol (DHCP), and other protocols to locate
specific computers on the network
3.
Bridges propagate all broadcast packets without filtering them.
4.
The network segments on either side of a bridge are in the same
broadcast domain, and are therefore still part of the same LAN.
Transparent bridging
1.
Transparent bridging is a method for automatically compiling a bridge’s
address tables.
2.
How do bridges know which computers are located on each network
segment?
a.
Bridges maintain an address table for each of the network segments.
b.
After reading the source and destination addresses of each packet, the
bridge compares them to the tables to determine whether to propagate
them.
3.
Where do the address tables come from?
a.
Originally, administrators had to create the tables manually.
b.
Today’s bridges use a technique called transparent bridging to
compile their own address tables.
c.
As the bridge processes a packet, it adds its source address to the
table for the segment from which it arrived.
d.
When a bridge lacks the information needed to filter a packet, it
propagates the packet by default.
Spanning tree algorithm (STA)
1.
STA is a protocol that bridges use to select one of the bridges to process
packets.
2.
Using multiple bridges to connect two network segments provides fault
tolerance.
3.
Redundancy can cause data loss when bridges have conflicting
information in their tables.
Outline, Chapter 3
Network+ Certification, Second Edition
4.
I.
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3.
Bridge Types
A.
B.
C.
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Multiple bridges can forward broadcasts endlessly, which is called a
bridge loop.
5.
The STA lets the bridges work together to select one bridge to function,
while the others remain idle.
Source route bridging
1.
Token Ring systems use source route bridging instead of transparent
bridging and the STA.
2.
Workstations themselves select the bridge they will use.
a.
Each computer transmits an All Rings Broadcast (ARB) message.
b.
Each bridge receiving the ARB adds a route designator to it,
identifying the bridge and port through which it received the packet.
c.
Each computer receiving an ARB returns it to its source.
3.
Bridges use the route designators to avoid sending packets to the same
bridge twice.
4.
The original source system uses the returned packets to determine which
bridge provides the most efficient route through the network to a given
destination.
D.
The three bridge types are local, translation, and remote.
Local bridge
1.
Connects network segments of the same type and at the same location
2.
Does not modify the data in the packets; it simply passes on the packets
or discards them
Translation bridge
1.
Connects network segments at the same location that use different media
or different protocols
2.
Strips the data-link layer frame off the packets and packages them in a
new frame for transmission on the other segment
3.
Disadvantages:
a.
Slower than local bridging
b.
Hardware more expensive
4.
Rarely used
Remote bridge
1.
Connects network segments at remote locations, using a wide area
network (WAN) link
2.
Advantage: reduces the amount of traffic passing over the WAN link
Chapter 3, Lesson 2
Switching
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1.
Switch Characteristics
A.
B.
C.
Data-link layer device
Replaces bridges, and in many cases, routers
Similar in appearance to a hub
Outline, Chapter 3
Network+ Certification, Second Edition
3
1.
D.
E.
F.
G.
H.
2.
Installing Switches
A.
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B.
C.
D.
E.
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4
Some manufacturers make hubs and switches that are identical except
for their markings.
A switch forwards incoming traffic out to the destination port only, as
opposed to a hub, which forwards traffic out through all ports.
Converts a shared network medium to a dedicated one
1.
Each packet takes a dedicated path from the source computer to the
destination.
2.
Each pair of computers forms its own collision domain.
3.
The switch forwards broadcast messages out through all ports, but not
unicasts or multicasts.
4.
No computers receive messages destined for other systems.
5.
No collisions occur during unicast transmissions.
Advantages
1.
Reduces network traffic congestion more efficiently than a bridge does
2.
Allows each pair of computers to use the full bandwidth of the network
Some switches can operate in full-duplex mode.
1.
Computers can transmit and receive at the same time.
2.
Effectively doubles the bandwidth of the network
In general, switches are more expensive than hubs and less expensive
than routers.
Switches are typically used to replace routers and bridges.
1.
A switch generally is not needed to replace a hub on a small LAN.
Replacing the routers on a backbone internetwork with switches can
dramatically reduce traffic congestion.
1.
On a routed network, the backbone carries all Internet traffic.
2.
On a switched network, any computer can open a dedicated connection
to any other computer.
3.
Collisions are dramatically decreased.
It is not necessary to replace all of the routers on a network with
switches at once.
1.
You can replace either a workgroup hub or a backbone router with a
switch, depending on whether the network has more intranetwork or
internetwork traffic.
Replacing routers with switches on an internetwork creates one large
broadcast domain.
1.
Each computer must process more unnecessary broadcasts.
There are several ways to address the broadcast problem, including
1.
Virtual LANs (VLANs)
a.
A VLAN is a subnet on a switched network that exists only in the
switches.
b.
You specify the addresses of computers that belong in a specific
subnet.
Outline, Chapter 3
Network+ Certification, Second Edition
c.
2.
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3.
Computers in a VLAN can be located anywhere because the subnet is
independent of the physical network.
d.
Broadcasts transmitted by a computer go only to the other computers
in the VLAN.
e.
All communications within a VLAN are switched.
f.
Communications between subnets can be routed or switched.
Layer 3 switching
a.
A variation on the VLAN concept that minimizes the amount of
routing needed between the VLANs
b.
When communication between systems on different VLANs is
required, a router establishes a connection between the systems and
then the switches take over.
c.
Routing occurs only when absolutely necessary.
Switch Types
A.
B.
Cut-through switch
1.
Forwards packets immediately
a.
Does not wait for the entire packet to arrive
2.
Most cut-through switches use a technique called matrix switching or
crossbar switching.
a.
Uses a grid of input/output (I/O) circuits that enable data to enter and
leave the switch through any port
3.
Advantages:
a.
Relatively inexpensive
b.
Minimizes latency (the delay that occurs when the switch processes
the packets)
Store-and-forward switch
1.
Waits until the entire packet arrives before forwarding it
2.
Can be either
a.
A shared-memory switch, which uses a common memory buffer to
store incoming data from all ports, or
b.
A bus architecture switch, which uses individual buffers for each
port, connected by a bus
3.
While the data is buffered, the switch
a.
Performs a cyclical redundancy check (CRC)
b.
Checks for data-link layer protocol faults such as runts, giants, and
jabber
4.
Disadvantages:
a.
Slower
b.
More expensive than cut-through switching
Outline, Chapter 3
Network+ Certification, Second Edition
5
Chapter 3, Lesson 3
Routing
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1.
Router Characteristics
A.
B.
C.
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2.
Router Functions
A.
B.
C.
3.
C.
6
Routers forward packets to the destination network.
1.
When a computer on a LAN wants to transmit data to a computer on
another LAN, the system sends its packets to a router on the local
network and the router forwards them to the destination network.
2.
If the destination system is not located on an adjacent network, the
router must forward the packets to another router.
3.
On a large internetwork, such as the Internet, packets might pass through
a dozen or more routers on the way to their destination.
Routers strip the data-link layer frame off incoming packets and add a
new frame for the outgoing network interface.
Routers fragment packets when necessary, depending on the
properties of the outgoing network.
Packet Routing
A.
B.
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Routers are hardware or software devices that connect networks to
form an internetwork.
1.
Are network layer devices
2.
Connect LANs running different data-link layer protocols, as long as
they are running the same network-layer protocol
IP performs most of the routing on networks today.
Transmission Control Protocol/Internet Protocol (TCP/IP) documents
often use the term “gateway” when referring to a router.
1.
A gateway can also refer to a hardware or software device that operates
at the application layer to provide an interface between two programs.
D.
Routers do not forward broadcasts, except in specific circumstances.
Routers forward packets based on the destination address in the
network layer protocol header.
Routers use routing tables to determine where to send packets.
1.
Every router has an internal table called a routing table, which contains
information about nearby networks.
2.
When a packet is addressed to a computer on a network the router is
directly connected to, the router transmits it to the destination.
3.
When a packet is addressed to a computer on a distant network, the
router looks up the destination in its routing table to locate an
appropriate router for the next hop.
4.
Routers select the most efficient path to the destination.
Routing process example
1.
The computers on each segment use the router connecting the segment
to the backbone as their default gateway.
Outline, Chapter 3
Network+ Certification, Second Edition
2.
E.
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F.
4.
Routing to the Internet
A.
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5.
The computers transmit all of the packets they generate to either a
specific system on the local network or the default gateway.
3.
The default gateway router strips the data-link layer frame off each packet
and reads the destination address from its network layer protocol header.
4.
Using the information in its routing table, the gateway determines which
router it must use to access the network the destination system is located on.
5.
The gateway then constructs a new frame for the packet, using the
backbone’s data-link layer protocol and specifying the router leading to
the destination network as the data-link layer destination address.
6.
When the packet reaches the next router, the process repeats itself,
except that the next router’s table indicates that the destination system is
on the segment the router is attached to.
a.
The router can therefore construct a frame that transmits the packet
directly to the destination system.
Each router that processes a packet is referred to as a hop.
1.
The efficiency of a given path through the internetwork is often
calculated by the number of hops required to reach the destination.
Routers also can connect networks at remote locations by using WAN
links.
1.
Because only packets destined for the other network pass over the WAN,
WAN traffic is minimized.
The most common use for WAN routers is connecting a network to an
Internet service provider (ISP), giving the computers on the network
access to the Internet.
1.
To connect your LAN to the Internet, you install a router that can
connect to an ISP, using any type of WAN link.
2.
The router is configured to forward all traffic not destined for the local
network to the ISP, which relays it to the Internet.
Understanding Routing Tables
A.
B.
C.
Routing tables are essential to the router operation and functions.
1.
Unlike bridges and switches, routers cannot compile their own routing
tables from the incoming packets.
Static routing
1.
Manual creation of a routing table by an administrator
2.
Practical on small networks only
a.
On a large network, static routing would require too much work.
3.
Cannot accommodate changes in network status
Dynamic routing
1.
Automatic creation of routing table entries by using a specialized routing
protocol
a.
Routers use the specialized protocols to exchange routing table
information.
Outline, Chapter 3
Network+ Certification, Second Edition
7
b.
2.
6.
Routing Metrics
A.
B.
C.
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7.
B.
C.
Routers can be stand-alone hardware devices or software running on a
computer.
Hardware routers
1.
Large enterprise networks use expensive rack-mounted routers to
connect LANs into an internetwork.
2.
Lower-priced router devices enable you to connect a small network to
the Internet.
Software routers include
1.
Microsoft Windows 2000 Internet Connection Sharing (ICS)
2.
Windows 2000 Routing and Remote Access
Chapter Summary
A.
B.
C.
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Networks often have more than one route to a particular destination.
Routers use a metric to evaluate the efficiency of each route.
Routers choose the most efficient route available by selecting the one
with the lowest metric.
1.
The metric can be based on the number of hops or on more complex
computations.
2.
The lowest metric is best.
Router Types
A.
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Routers have direct information only about the networks they are
connected to.
c.
Sharing information with other routers enables all of the routers to
build a picture of the network.
Routing protocols enable routers to
a.
Build their tables automatically, with no participation by an
administrator
b.
Update their tables when the network changes, such as when a router
malfunctions
Bridging
1.
Bridges selectively relay packets between network segments, depending
on their data-link layer destination addresses.
2.
Bridges maintain a single broadcast domain and create separate collision
domains.
Switching
1.
Switches improve on the function of bridges by forwarding packets only
to their destination systems.
2.
Switches reduce the collisions on a network and increase the bandwidth
available to each computer.
Routing
1.
Routers are used to connect networks together at the network layer of the
OSI reference model.
Outline, Chapter 3
Network+ Certification, Second Edition
2.
3.
Routers strip away the data-link layer frame of incoming packets and
build a new frame using the data-link layer protocol of the outgoing
network.
Routing tables can be created manually by a network administrator or
automatically by a routing protocol.
Outline, Chapter 3
Network+ Certification, Second Edition
9