From a network efficiency standpoint, it is easy to
see that it is much better for the network when the
switch knows all the addresses on every port.
However, it is not always practical to enter this
information manually. As the network grows and
changes are made, it becomes almost impossible to
keep up.
A switch always does something when it receives
traffic. The preference is to send the traffic out a
specific port (called filtering), but this works only
when the location of the intended destination is
known. When the destination address is unknown,
the switch forwards the traffic out every port,
except the one on which the traffic was received.
This process is called flooding. Think of this as a
guy calling every number in the phone book because
he lost a woman’s number from the night before. 61
IT Certification CCIE,CCNP,CCIP,CCNA,CCSP,Cisco Network Optimization and Security Tips
Broadcast and Collision Domains
From time to time, a device on the network will
want to communicate with all other “local” devices
at the same time. Typically, this occurs when a
device wants to query the network for an IP address,
when a device is newly added to a network, or when
a change occurs in the network.
A group of devices that receive all broadcast messages
from members of that group is called a
broadcast domain. Network broadcast domains
typically are segmented with Layer 3 devices (routers).
Think of a broadcast domain as like standing in
your yard and yelling as loudly as you can. The
neighbors who hear you are within your broadcast
domain.
want to communicate with all other “local” devices
at the same time. Typically, this occurs when a
device wants to query the network for an IP address,
when a device is newly added to a network, or when
a change occurs in the network.
A group of devices that receive all broadcast messages
from members of that group is called a
broadcast domain. Network broadcast domains
typically are segmented with Layer 3 devices (routers).
Think of a broadcast domain as like standing in
your yard and yelling as loudly as you can. The
neighbors who hear you are within your broadcast
domain.
Broadcast and Collision Domains
From time to time, a device on the network will
want to communicate with all other “local” devices
at the same time. Typically, this occurs when a
device wants to query the network for an IP address,
when a device is newly added to a network, or when
a change occurs in the network.
A group of devices that receive all broadcast messages
from members of that group is called a
broadcast domain. Network broadcast domains
typically are segmented with Layer 3 devices (routers).
Think of a broadcast domain as like standing in
your yard and yelling as loudly as you can. The
neighbors who hear you are within your broadcast
domain.
want to communicate with all other “local” devices
at the same time. Typically, this occurs when a
device wants to query the network for an IP address,
when a device is newly added to a network, or when
a change occurs in the network.
A group of devices that receive all broadcast messages
from members of that group is called a
broadcast domain. Network broadcast domains
typically are segmented with Layer 3 devices (routers).
Think of a broadcast domain as like standing in
your yard and yelling as loudly as you can. The
neighbors who hear you are within your broadcast
domain.
What Problems Need to Be Solved?
• MAC address learning: Switches must learn
about the network to make intelligent decisions.
Because of the size and changing nature of networks,
switches have learned how to discover
network addresses and keep track of network
changes. Switches do this by finding the address
information contained in the frames flowing
through the network, and they maintain private
tables with that information.
• Forwarding and filtering: Switches must decide
what to do with traffic. These decisions are
based on the switch’s knowledge of the network.
• Segmenting end stations: Switches must also have
mechanisms for segregating users into logical
groupings (virtual LANs [VLAN] or broadcast
domains) to allow efficient provisioning of service.
about the network to make intelligent decisions.
Because of the size and changing nature of networks,
switches have learned how to discover
network addresses and keep track of network
changes. Switches do this by finding the address
information contained in the frames flowing
through the network, and they maintain private
tables with that information.
• Forwarding and filtering: Switches must decide
what to do with traffic. These decisions are
based on the switch’s knowledge of the network.
• Segmenting end stations: Switches must also have
mechanisms for segregating users into logical
groupings (virtual LANs [VLAN] or broadcast
domains) to allow efficient provisioning of service.
Why Should I Care About Switching?
Advances in switching technology combined with
a decrease in switch prices have made computer
networks a common and increasingly important
aspect of business today.
a decrease in switch prices have made computer
networks a common and increasingly important
aspect of business today.
Switches Take Over the World
As switches established themselves in networks, vendors added increasing
functionality. Modern switches can perform forwarding decisions based on
Layer 3 routing, as well as on Layer 4 and above. Even though switches can
perform the functions of other higher-layer devices such as routers and content
switches, you still must separate these functionalities to avoid single points of
failure. Switches are the workhorse of networks, providing functionality across
almost all layers of the OSI model reliably and quickly. Switches can also provide
power to devices such as IP-based phones using the same Ethernet connection.
Again, this applies to very large switches serving corporate networks
rather than the switches in a small office or home.
functionality. Modern switches can perform forwarding decisions based on
Layer 3 routing, as well as on Layer 4 and above. Even though switches can
perform the functions of other higher-layer devices such as routers and content
switches, you still must separate these functionalities to avoid single points of
failure. Switches are the workhorse of networks, providing functionality across
almost all layers of the OSI model reliably and quickly. Switches can also provide
power to devices such as IP-based phones using the same Ethernet connection.
Again, this applies to very large switches serving corporate networks
rather than the switches in a small office or home.
Switching Ethernets
As switch Ethernet ports became less expensive, switches replaced hubs in the
wiring closet. Initially, when switches were introduced, network administrators
plugged hubs (containing multiple hosts) into switch ports. But eventually, it
became cost-effective to plug the hosts directly into a switch port. This
arrangement gives each host its own dedicated Ethernet and removes the possibility
of collisions. Because a dedicated switch connection has only two devices
(the switch and the host), you can configure an Ethernet switch port as full
duplex. This means that a device can receive incoming traffic and transmit
traffic simultaneously. End stations have considerably more bandwidth when
they use switches. Ethernet can run at multiple speeds: 10 Mbps, 100 Mbps,
1 Gbps, and 10 Gbps. Therefore, switches can provide connectivity at these
speeds. However, network applications and the web create considerably more
network traffic, reintroducing new congestion problems. Switches can use
quality of service (QoS) and other mechanisms to help solve the congestion
issue.
wiring closet. Initially, when switches were introduced, network administrators
plugged hubs (containing multiple hosts) into switch ports. But eventually, it
became cost-effective to plug the hosts directly into a switch port. This
arrangement gives each host its own dedicated Ethernet and removes the possibility
of collisions. Because a dedicated switch connection has only two devices
(the switch and the host), you can configure an Ethernet switch port as full
duplex. This means that a device can receive incoming traffic and transmit
traffic simultaneously. End stations have considerably more bandwidth when
they use switches. Ethernet can run at multiple speeds: 10 Mbps, 100 Mbps,
1 Gbps, and 10 Gbps. Therefore, switches can provide connectivity at these
speeds. However, network applications and the web create considerably more
network traffic, reintroducing new congestion problems. Switches can use
quality of service (QoS) and other mechanisms to help solve the congestion
issue.
Switching Basics: It’s a Bridge
Network devices have one primary purpose: to pass network traffic from one
segment to another. (There are exceptions, of course, such as network analyzers,
which inspect traffic as it goes by.) With devices that independently make
forwarding decisions, traffic can travel from its source to the destination. The
higher up the OSI model a device operates, the deeper it looks into a packet to
make a forwarding decision. Railroad-switching stations provide a similar
example. The switches enable a train to enter the appropriate tracks (path)
that take it to its final destination. If the switches are set wrong, a train can
end up traveling to the wrong destination or traveling in a circle.
Switching technology emerged as the replacement for bridging. Switches provide
all the features of traditional bridging and more. Compared to bridges,
switches provide superior throughput performance, higher port density, and
lower per-port cost.
The different types of bridging include the following:
• Transparent bridging primarily occurs in Ethernet networks.
• Source-route bridging occurs in Token Ring networks.
• Translational bridging occurs between different media. For example, a translational
bridge might connect a Token Ring network to an Ethernet network.
Bridging and switching occur at the data link layer (Layer 2 in the OSI model),
which means that bridges control data flow, provide transmission error handling,
and enable access to physical media. Basic bridging is not complicated:
A bridge or switch analyzes an incoming frame, determines where to forward
the frame based on the packet’s header information (which contains information
on the source and destination addresses), and forwards the frame toward
its destination. With transparent bridging, forwarding decisions happen one
hop (or network segment) at a time. With source-route bridging, the frame
contains a predetermined path to the destination.
Bridges and switches divide networks into smaller, self-contained units.
Because only a portion of the traffic is forwarded, bridging reduces the overall
traffic that devices see on each connected network. The bridge acts as a kind
of firewall in that it prevents frame-level errors from propagating from one
segment to another. Bridges also accommodate communication among more
devices than are supported on a single segment or ring.
Bridges and switches essentially extend the effective length of a LAN, permitting
more workstations to communicate with each other within a single broadcast
domain. The primary difference between switches and bridges is that
bridges segment a LAN into a few smaller segments. Switches, through their
increased port density and speed, permit segmentation on a much larger scale.
Modern-day switches used in corporate networks have hundreds of ports per
chassis (unlike the four-port box connected to your cable modem).
Additionally, modern-day switches interconnect LAN segments operating at
different speeds.
Switching describes technologies that are an extension of traditional bridges.
Switches connect two or more LAN segments and make forwarding decisions
about whether to transmit packets from one segment to another. When a
frame arrives, the switch inspects the destination and source Media Access
Control (MAC) addresses in the packet. (This is an example of store-andforward
switching.) The switch places an entry in a table indicating that the
source MAC address is located off the switch interface on which the packet
arrived. The bridge then consults the same table for an entry for the destination
MAC address. If it has an entry for the destination MAC address, and the
entry indicates that the MAC address is located on a different port from which
the packet was received, the switch forwards the frame to the specified port.
If the switch table indicates that the destination MAC address is located on the
same interface on which the frame was just received, the bridge does not forward
the frame. Why send it back onto the network segment from which it
came? This decision is where a switch reduces network congestion. Finally, if
the destination MAC address is not in the switch’s table, this indicates that the
switch has not yet seen a frame destined for this MAC address. In this case,
the switch then forwards the frames out all other ports (called flooding) except
the one on which the packet was received.
At their core, switches are multiport bridges. However, switches have radically
matured into intelligent devices, replacing both bridges and hubs. Switches not
only reduce traffic through the use of bridge tables, but also offer new functionality
that supports high-speed connections, virtual LANs, and even traditional
routing.
segment to another. (There are exceptions, of course, such as network analyzers,
which inspect traffic as it goes by.) With devices that independently make
forwarding decisions, traffic can travel from its source to the destination. The
higher up the OSI model a device operates, the deeper it looks into a packet to
make a forwarding decision. Railroad-switching stations provide a similar
example. The switches enable a train to enter the appropriate tracks (path)
that take it to its final destination. If the switches are set wrong, a train can
end up traveling to the wrong destination or traveling in a circle.
Switching technology emerged as the replacement for bridging. Switches provide
all the features of traditional bridging and more. Compared to bridges,
switches provide superior throughput performance, higher port density, and
lower per-port cost.
The different types of bridging include the following:
• Transparent bridging primarily occurs in Ethernet networks.
• Source-route bridging occurs in Token Ring networks.
• Translational bridging occurs between different media. For example, a translational
bridge might connect a Token Ring network to an Ethernet network.
Bridging and switching occur at the data link layer (Layer 2 in the OSI model),
which means that bridges control data flow, provide transmission error handling,
and enable access to physical media. Basic bridging is not complicated:
A bridge or switch analyzes an incoming frame, determines where to forward
the frame based on the packet’s header information (which contains information
on the source and destination addresses), and forwards the frame toward
its destination. With transparent bridging, forwarding decisions happen one
hop (or network segment) at a time. With source-route bridging, the frame
contains a predetermined path to the destination.
Bridges and switches divide networks into smaller, self-contained units.
Because only a portion of the traffic is forwarded, bridging reduces the overall
traffic that devices see on each connected network. The bridge acts as a kind
of firewall in that it prevents frame-level errors from propagating from one
segment to another. Bridges also accommodate communication among more
devices than are supported on a single segment or ring.
Bridges and switches essentially extend the effective length of a LAN, permitting
more workstations to communicate with each other within a single broadcast
domain. The primary difference between switches and bridges is that
bridges segment a LAN into a few smaller segments. Switches, through their
increased port density and speed, permit segmentation on a much larger scale.
Modern-day switches used in corporate networks have hundreds of ports per
chassis (unlike the four-port box connected to your cable modem).
Additionally, modern-day switches interconnect LAN segments operating at
different speeds.
Switching describes technologies that are an extension of traditional bridges.
Switches connect two or more LAN segments and make forwarding decisions
about whether to transmit packets from one segment to another. When a
frame arrives, the switch inspects the destination and source Media Access
Control (MAC) addresses in the packet. (This is an example of store-andforward
switching.) The switch places an entry in a table indicating that the
source MAC address is located off the switch interface on which the packet
arrived. The bridge then consults the same table for an entry for the destination
MAC address. If it has an entry for the destination MAC address, and the
entry indicates that the MAC address is located on a different port from which
the packet was received, the switch forwards the frame to the specified port.
If the switch table indicates that the destination MAC address is located on the
same interface on which the frame was just received, the bridge does not forward
the frame. Why send it back onto the network segment from which it
came? This decision is where a switch reduces network congestion. Finally, if
the destination MAC address is not in the switch’s table, this indicates that the
switch has not yet seen a frame destined for this MAC address. In this case,
the switch then forwards the frames out all other ports (called flooding) except
the one on which the packet was received.
At their core, switches are multiport bridges. However, switches have radically
matured into intelligent devices, replacing both bridges and hubs. Switches not
only reduce traffic through the use of bridge tables, but also offer new functionality
that supports high-speed connections, virtual LANs, and even traditional
routing.
Fast Computers Need Faster Networks
The PC emerged as the most common desktop computer in the 1980s. LANs
emerged as a way to network PCs in a common location. Networking technologies
such as Token Ring and Ethernet allowed users to share resources
such as printers and exchange files with each other. As originally defined,
Ethernet and Token Ring provided network access to multiple devices on the
same network segment or ring. These LAN technologies have inherent limitations
as to how many devices can connect to a single segment, as well as the
physical distance between computers. Desktop computers got faster, the number
of computers grew, operating systems began multitasking (allowing multiple
tasks to operate at the same time), and applications became more networkcentric.
All these advancements resulted in congestion on LANs.
To address these issues, two device types emerged: repeaters and bridges.
Repeaters are simple Open Systems Interconnection (OSI) Layer 1 devices that
allow networks to extend beyond their defined physical distances (which were
limited to about 150 feet without the use of a repeater).
Bridges are OSI Layer 2 devices that physically split a segment into two and
reduce the amount of traffic on either side of the bridge. This setup allows
more devices to connect to the LAN and reduces congestion. LAN switches
emerged as a natural extension of bridging, revolutionizing the concept of
local-area networking.
emerged as a way to network PCs in a common location. Networking technologies
such as Token Ring and Ethernet allowed users to share resources
such as printers and exchange files with each other. As originally defined,
Ethernet and Token Ring provided network access to multiple devices on the
same network segment or ring. These LAN technologies have inherent limitations
as to how many devices can connect to a single segment, as well as the
physical distance between computers. Desktop computers got faster, the number
of computers grew, operating systems began multitasking (allowing multiple
tasks to operate at the same time), and applications became more networkcentric.
All these advancements resulted in congestion on LANs.
To address these issues, two device types emerged: repeaters and bridges.
Repeaters are simple Open Systems Interconnection (OSI) Layer 1 devices that
allow networks to extend beyond their defined physical distances (which were
limited to about 150 feet without the use of a repeater).
Bridges are OSI Layer 2 devices that physically split a segment into two and
reduce the amount of traffic on either side of the bridge. This setup allows
more devices to connect to the LAN and reduces congestion. LAN switches
emerged as a natural extension of bridging, revolutionizing the concept of
local-area networking.
What They Gave Away
In the 1970s Xerox Corporation assembled a
group of talented researchers to investigate new
technologies. The new group was located in the
newly opened Palo Alto Research Center (PARC),
well away from the corporate headquarters in
Connecticut.
In addition to developing Ethernet, the brilliant
folks at the PARC invented the technology for
what eventually became the personal computer
(PC), the graphical user interface (GUI), laser printing,
and very-large-scale integration (VLSI).
Inexplicably, Xerox Corporation failed to recognize
the brilliance (and commercial viability) of many of
these homegrown innovations and let them walk
out the door.
To give you an idea of what this cost Xerox in
terms of opportunity, the worldwide budget for
Ethernet equipment was more than $7 billion in
2006 and was expected to grow to more than $10
billion by 2009. Just imagine if a single company
owned the assets of Apple, Intel, Cisco, HP, and
Microsoft. There almost was such a company. Its
name is Xerox.
At-a-Glance: Ethernet
group of talented researchers to investigate new
technologies. The new group was located in the
newly opened Palo Alto Research Center (PARC),
well away from the corporate headquarters in
Connecticut.
In addition to developing Ethernet, the brilliant
folks at the PARC invented the technology for
what eventually became the personal computer
(PC), the graphical user interface (GUI), laser printing,
and very-large-scale integration (VLSI).
Inexplicably, Xerox Corporation failed to recognize
the brilliance (and commercial viability) of many of
these homegrown innovations and let them walk
out the door.
To give you an idea of what this cost Xerox in
terms of opportunity, the worldwide budget for
Ethernet equipment was more than $7 billion in
2006 and was expected to grow to more than $10
billion by 2009. Just imagine if a single company
owned the assets of Apple, Intel, Cisco, HP, and
Microsoft. There almost was such a company. Its
name is Xerox.
At-a-Glance: Ethernet
LAN Routers
LAN-based routers greatly extend the speed, distance,
and intelligence of Ethernet LANs. Routers
also allow traffic to be sent along multiple paths.
Routers, however, require a common protocol
between the router and end stations.
and intelligence of Ethernet LANs. Routers
also allow traffic to be sent along multiple paths.
Routers, however, require a common protocol
between the router and end stations.
Switched Ethernet
A LAN switch can be thought of as a high-speed,
multiport bridge with a brain. Switches don’t just
allow each end station to have a dedicated port
(meaning that no collisions occur). They also allow
end stations to transmit and receive at the same
time (using full duplex), greatly increasing the
LAN’s efficiency.
multiport bridge with a brain. Switches don’t just
allow each end station to have a dedicated port
(meaning that no collisions occur). They also allow
end stations to transmit and receive at the same
time (using full duplex), greatly increasing the
LAN’s efficiency.
Bridges
Bridges are simple Layer 2 devices that create new
segments, resulting in fewer collisions. Bridges
must learn the addresses of the computers on each
segment to avoid forwarding traffic to the wrong
port. Unlike hubs, which are usually used for networks
with a small number of end stations (4 to
8), bridges can handle much larger networks with
dozens of end stations.
segments, resulting in fewer collisions. Bridges
must learn the addresses of the computers on each
segment to avoid forwarding traffic to the wrong
port. Unlike hubs, which are usually used for networks
with a small number of end stations (4 to
8), bridges can handle much larger networks with
dozens of end stations.
Hubs
Hubs enable you to add and remove computers
without disabling the network, but they do not
create additional collision domains.
without disabling the network, but they do not
create additional collision domains.
Ethernet Segments
A segment is the simplest form of network, in
which all devices are directly connected. In this type
of arrangement, if any of the computers gets disconnected,
or if one is added, the segment is disabled.
which all devices are directly connected. In this type
of arrangement, if any of the computers gets disconnected,
or if one is added, the segment is disabled.
Increasing Bandwidth
In addition to creating additional segments to
increase available bandwidth, you can use a faster
medium such as optical fiber or Gigabit Ethernet.
Although these technologies are faster, they are still
shared media, so collision domains will still exist
and will eventually experience the same problems
as slower media.
increase available bandwidth, you can use a faster
medium such as optical fiber or Gigabit Ethernet.
Although these technologies are faster, they are still
shared media, so collision domains will still exist
and will eventually experience the same problems
as slower media.
Increasing Bandwidth
In addition to creating additional segments to
increase available bandwidth, you can use a faster
medium such as optical fiber or Gigabit Ethernet.
Although these technologies are faster, they are still
shared media, so collision domains will still exist
and will eventually experience the same problems
as slower media.
increase available bandwidth, you can use a faster
medium such as optical fiber or Gigabit Ethernet.
Although these technologies are faster, they are still
shared media, so collision domains will still exist
and will eventually experience the same problems
as slower media.
Increasing Bandwidth
In addition to creating additional segments to
increase available bandwidth, you can use a faster
medium such as optical fiber or Gigabit Ethernet.
Although these technologies are faster, they are still
shared media, so collision domains will still exist
and will eventually experience the same problems
as slower media.
increase available bandwidth, you can use a faster
medium such as optical fiber or Gigabit Ethernet.
Although these technologies are faster, they are still
shared media, so collision domains will still exist
and will eventually experience the same problems
as slower media.
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