Download Overview of Switches

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Introduction
Switches are widely being used today by most of the business networks to connect multiple
devices such as computers, printers and servers that are present within a particular
boundary like a campus or building. The switch is responsible for enabling communication
between the networking devices on a Local area network (LAN) so that they could efficiently
talk to each other. The greater advantage of having them in the business is that they play a
primary role in sharing the information and allocating the resources. The home networks
made use of this little device before the routers popularized.
In various types of networks the capability of switching exists. The widely known and the
commonly used type are the Ethernet Switches. There are several models of these switches
based on performance factors. They support a variety of connected devices and operate at
the Data link layer of the OSI Model. Corporate switches can support from 32 to 128
connections, whereas the consumer grade switches can support 4 to 8 connections. Multiple
switches could be connected to each other to provide larger device connectivity on the
network.
How they work?
Hubs and Switches are look identical to each other and also occupy the same place as hubs,
but their function varies. A switch unlike hub can inspect data in each packet as it is being
received through the packet switching method instead of sending the same signal to every
port. Through the packet examination it finds out the source and the destination details of
every packet and forwards the packets accordingly to their destination. This process
ultimately conserves a lot of network bandwidth and improves the performance when
compared to hubs.
The brief note points of what it does are given below.
 Once the data packet reaches a switch, it gathers all the information about the
packet by inspecting it.
 The information includes the sources and the destination details of the packet.
 It maps the address to a table of addresses of the nodes on the network segments to
forward it.
 If the network segments are same, then the packet is dropped or filtered. If they are
different then it is forwarded to its specific network segment.
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 Only after mapping, that specific traffic is allowed to pass through it and forwarded
to its network segment.
 The bad traffic is filtered out and stopped from spreading on the network.
The basic functions of the switch are divided into three phases;
Learning: This is process where the MAC addresses of the devices that are connected are
obtained.
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Frame reaches the port of a switch
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The MAC address of the source is read by the switch from the frame
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MAC address compared with the CAM table that contains a list of MAC addresses.
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If MAC address entry is found on the CAM table, the incoming port and the port on
the MAC table are compared by the switch
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If the port numbers do not match, the port number entry on the table is updated
with the new port number.
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If MAC address not found on the CAM table, the address is added to the table along
with the port number from the information on the frame.
Forwarding: The process where the traffic is forwarded from one device to another device
connected on different ports on the switch.
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From the learning process, the switch updates its CAM table and has all the required
information extracted from the received Ethernet frame to forward the packet. The
destination MAC address is also learnt as it is necessary for forwarding the data. The
MAC address of the destination is required to find out the port number of the
destination where the device is connected.
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If the MAC address of the destination is found on the table, the frame is forwarded
through the port number corresponding to the address.
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If the MAC address is not found on the table, the frame is forwarded to all the ports
on that switch leaving the source port. This process is called flooding. Lots of traffic
is lost in this process of learning the destination MAC address. When the destination
gets the frame, it sends back a reply frame to the device at the source.
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The MAC address from where the reply was sent is learnt by the switch and it adds
that address to the MAC address table.
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If the source and the destination MAC addresses are the same, the frame is dropped
by the switch. This process is called filtering. This happens when a hub is connected
to a port on the switch and when both the source and the destination devices are
connected to that hub.
Preventing L2 loops
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In real time LAN networking, to avoid the failure of the network redundant links are
created in cases where a link fails.
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These redundant links could causing L2 looping and broadcast storms.
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This function on the switch prevents L2 looping and broadcast storms from
happening.
Some Important Points
 Packet Filtering and Forwarded packet regeneration make the switching technology
more efficient. The network is split into separate independent collision domains.
Packet regeneration allows more number of nodes to be used on the network and
also reduces the collision rates overall on the network.
 In the switched network, every segment is a collision domain. This allows parallelism
where half of computers connected to it can send data simultaneously. Shared
networks have all the nodes on one collision domain that is shared.
 Switches could be installed very easily. They build a table of addresses on every
segment, as they determine them from the packets that pass through them.
 Different types of networks could be connected through a switch. For example
Ethernet and Fast Ethernet could be connected or the same type of networks could
be connected through it. Fast Ethernet links which are high speed links are offered
by switches today to link multiple switches or to add more bandwidth to get the
traffic under control at the server. Such networks where multiple switches are linked
through fast Ethernet links are called collapsed backbone networks.
 A full segment can be utilized for one node in order to gain higher speeds. Nodes to
which there is high flow of traffic could be connected to a port that is dedicated for it
on that switch.
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Types of Ethernet Switches
There are two categories of Ethernet Switches
Modular Switches: These switches allow us to add modules into them to expand their usage
and to make them more flexible for use on the networks where addresses keep changing.
These expansion modules are specific to applications. Some examples of these modules are
Network Analysis, Firewall, cooling fans etc. Cisco catalyst 4000 series and 6000 series are
best examples for Modular Switches.
Fixed Configuration switches: These switches have ports of fixed number and hence cannot
be expanded. Cisco Catalyst 2000 series, 3000 series and Cisco 300/500 series are best
examples of these kind of switches. The fixed configuration switches are broken down
further into three categories;
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Unmanaged Switches: These switches are the best to deploy in real time as
they are cost effective and function on switching and connectivity on Layer 2.
They can be easily added to fit extra devices on the network. Some of these
switches are capable of diagnosing the cable, prioritizing the traffic with the
help of the QoS which is a default setting and also energy saving capability
using EEE and PoE. From the name of the switch type we can see that these
switches cannot be managed or modified. Unmanaged switches can be
plugged in and used right away as they do not require any prior
configurations to make them function. Best example of these kind of switches
is Cisco 100 series.
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Smart switches: These Switches are also called lightly managed switches.
They are very blurred and change very fast. They offer QoS, Security and
Management but at low levels and hence they are less scalable when
compared to the managed switches. These switches are the best alternatives
to the managed switches as they are cost effective. They function perfectly
when they are fit at the edges of large network which have managed switches
used at their core. They also fit best for small networks or low complexity
networks. They have a wide variety of capabilities. These devices have a
specific interface for management. Earlier these devices were managed and
configured through a web interface but now they are manageable through the
command line interface as well. When compared to the managed switches, its
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capabilities are lighter. But the management interface on the smart switch is
easier and simpler to use when compared to the managed switches. The
network could be segmented into workgroups by the creation of VLANs when
using this switch category. The segmentation is allowed though the number of
VLANs and nodes are low unlike what Managed switches offer. Certain levels
of security are also offered by smart switches such as 802.1x endpoint
authentication and Access control lists. This extent of security provided may
not be as much provided by the Managed switch. Basic QoS is supported by
the smart switches that help in user and application prioritization making
them versatile. The best example of this switch category is the Cisco 200
series switches.
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Fully Managed Layer2 and Layer3 switches: These switches provide the
most complex features as per their design. The features include the best
experience of application, high security, effective network control and
management, high scalability in fixed configuration switches. As a result of
these features, they are mostly deployed in larger networks as access
switches or in the small networks as core switches. These switches support
both layer 2 switching and layer 3 Routing. When seen from the security point
of view, these switches protect the data plane, where the user forwards the
traffic , the control plane, where the traffic is sent from one device to another
on the network to drive to the correct destination and the management plane,
where the network or device is managed by the traffic itself. Services like
storm control on networks, DoS protection and lots more. Dropping flexibly,
limiting the rate, mirroring, traffic logging by L2, L3 addresses, port numbers
of TCP/UDP, type of Ethernet, TCP flags or ICMP etc. Features are very rich in
this switch category. They enable the switches to fight and protect
themselves against the DoS attacks. Features include Dynamic inspection of
ARP, DHCP IPv4 snooping and lots more. Other security features include
private VLANs for securing user communities and isolation of the devices,
secured management, Policing the control plane for protecting the switch of
CPU AND wide support for 802.1x. These switches have tables of large size so
that VLANs can be created in large number, MAC table size, ACL policies, IP
routes etc. These are looked upon from the scalability point of view. To
provide high availability of network and uptime, these switches use Virtual
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Router Redundancy Protocol for supporting the Layer 3 redundancy. These
switches also provide support for large number of “Link aggregation groups”,
and protection for Layer 2 such as guards for Spanning tree root and BPDU.
Optimization of the multicast traffic on LAN, Congestion avoidance of TCP,
Limitation on traffic etc is the features of QoS.
The best example of this switch category is Cisco 300 series and 500 series.
Switches can also be chosen based on other options like;
 Speed
 Port Count
 POE vs Non POE
 Stackable vs Standalone
Things to be considered when selecting a switch;
Specifications like traffic flow details, data servers and storage servers should be on the top
priority to select the switch.
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Future growth
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Performance
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Port speed of the switch
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Port density
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Rate of forwarding
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Link aggregation
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Power over Ethernet(PoE)