Transcript Dell Presentation Template Standard 4:3 Layout

What is an Ethernet Switch?
Victor Lama’s Concept of the Week – 09/25/2010
G500-Fabric Specialist
Preface
• Folks, I know this topic seems very basic, but included in this
short tutorial are key foundational concepts that most nonnetworking IT professionals usually miss
• So, please read this through carefully and I am sure you will
get a lot out of it.
• Future COWs will build on these foundational topics.
• An icon of a key next to a dialogue box means – you guessed
it – it’s a key concept that you must grasp really well.
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Back in the Day….
• …computing resources and peripherals were networked by connecting them to a hub.
• The Ethernet hub is a device that operates at layer 1 of the OSI model. When it
receives an Ethernet frame on one of its ports, it simply replicates the string of bits
and floods them out all its ports, leaving it up to the end system to filter out unwanted
frames.
• Hubs are “dumb.” They have no clue what is attached to them because they do not
have the ability to read the layer 2 frame header and glean any useful information,
such as source and destination MAC addresses.
• Therefore, all the end-stations and the hub make up a single broadcast domain.
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So, Hubs Are Dumb – Big Deal!
The Ethernet frames get where they’re going, right?
• Ummmm….sometimes. Besides deluging the network with massive amounts of
unnecessary traffic, the network hub and cabling create a shared logical bus topology
– a single wire over which all LAN traffic travels.
• That means all the end stations are part of the same collision domain. The result is
that Ethernet frames sent over the same medium may collide with each other on the
wire and would have to be resent or be lost.
Resulting Topology When Hub is in Use
Single Collision Domain
Single Broadcast Domain
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How Does Ethernet Adapt to Hub-life?
• Each Ethernet end station utilizes a system called CSMA-CD. That stands for Carrier
Sense Multiple Access with Collision Detect.
• Before sending Ethernet frames, the station listens first for traffic, and if it does not
detect any electrical signals on the shared wire, it proceeds with sending the frame
out to the network.
• If the station is listening, it’s not sending. This is known as half-duplex transmissions.
• If after sending the frame a collision is detected, the transmitting station starts a
random back-off timer and resends when it expires.
• This makes for a slow network with a lot of packet flooding and collisions!
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Then Came Layer 2 Switches!
• Unlike hubs, switches are smart and by default operate at Layer 2 of the OSI model. As
a switch receives an Ethernet frame on one of its ports, it takes note of the source
MAC-address in the frame’s header and maps it to the switch port on which the
frame arrived. Then it adds the entry to a corresponding MAC-address table, which it
will use to forward Ethernet frames in the future.
• This puts an end to unnecessary flooding because the switch now has the
intelligence to know which egress port to use to reach a given MAC-address.
Broadcasts now become unicasts.
• Moreover, switches offer a point-to-point topology in which each switch port and
the host directly connected to it comprise a single collision domain. Each port is
electrically isolated from other ports, so collisions don’t occur. This allows for fullduplex communication between the switch port and the end station.
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I Said Unnecessary Flooding!
• Sometimes it is necessary for a switch to rely on frame flooding. For example, if a
switch receives an Ethernet frame with a destination MAC-address that is not found in
the switch’s MAC-address table, it floods the packet out all ports – except the one on
which it was received – and then waits for a response from the intended end station.
• When the intended end station responds, the switch takes note of which port the
response came in on and adds that packet’s source MAC-address to the MAC-address
table.
• This process is known as unknown unicast flooding.
• The underlying supposition that the Ethernet protocol makes is that all hosts on the
LAN are part of the same layer 2 broadcast domain. This is an important characteristic
of Ethernet that has ramifications with regard to its scalability. We will talk more about
that later.
• Take note that the switch must replicate the received frame as many times as it must
be sent as part of the broadcast. That equates to elevated CPU usage and possible
degradation of switch performance.
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Wait, It Gets Better!
• When a switch broadcasts an unknown unicast out all of its ports, it’s acting just like a
hub, where each end-station/switch port belongs to the same broadcast domain. On
a network with dozens or hundreds of hosts, that can really put the brakes on
communication!
• The solution that Ethernet switches offer to segment the broadcast domain into
smaller ones are VLANs! Virtual Local Area Networks.
• VLANs are Layer 2 (Data Link Layer) constructs that offer logical separation between
groups of different end stations. Each switch port is assigned to a certain VLAN and
each VLAN is its own broadcast domain. As a result, Ethernet frames that need to be
broadcast because the destination MAC-address is unknown (unknown unicast
flooding), will only be sent out the ports that belong to the VLAN that the source
host belongs to, and not out of every port on the switch. You can think of a VLAN as
a construct that imposes a logical boundary to the broadcast traffic.
• That could mean the difference between replicating a packet 50
times for 50 ports or 500 times for 500 ports!
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VLAN Illustration
Here we have 3 broadcast domains – VLANs 2, 3 and 4.
Broadcasts are confined to hosts on the same VLAN.
So, a broadcast sent out by a host on VLAN 4
will only be heard by hosts on VLAN 4.
Trunk must be configured to carry traffic tagged as belonging to VLAN 4
for VLAN 4 hosts on both switches to be able to communicate.
Benefits of VLANs:
•Smaller Broadcast Domains
•Microsegmentation of Collison Domains – one per link
•Security through Isolation
•Flexible Workgroups
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SUMMARY
• Hubs are “dumb” network appliances that operate at layer 1 of the OSI model. They do
not have the ability to read the header information of Ethernet frames.
• Hubs create a logical bus topology in which packet collisions occur and hosts need to
communicate in half-duplex mode.
• Switches are smart network appliances that glean information from the headers of
layer 2 Ethernet frames and build forwarding intelligence based on it.
• Switches allow the creation of layer 2 software constructs known as VLANs, where
each VLAN is its own broadcast domain.
• Switches unicast Ethernet frames to known destinations but flood frames out every
port in a VLAN when the destination host is unknown, including trunk ports.
I bet you’re asking yourself “Why did he put the words ‘including trunk ports’ in bold?”
TUNE IN Next Week for the answer! Wooohooohahahahaha!!!
(Wringing my hands in fiendish glee!)
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