1. Layer 3 Switches
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2. Topics Covered
► Definitions
► Characteristics
► Comparison between Routers and Layer 3 Switches
► Functionality
► The Benefits of Layer 3 Switching
► Cost
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3. Router
► A router is a computer
networking device that
buffers and forwards data
packets across an
internetwork toward their
destinations, through a
process known as routing.
► Routing occurs at layer 3
(Network layer )
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4. Network Switches
► A network switch
is a networking device
that performs
transparent
connection of
multiple network
segments with
forwarding based on
MAC addresses at up
to the speed of the
hardware.
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5. Layer 3 Switches
► A Layer 3 switch is a high-performance device for network routing. It is
relatively new term, was conceived as a technology to improve on the
performance of routers used in large local area networks (LANs).
► It can support the same routing protocols as network routers do. Both
inspect incoming packets and make dynamic routing decisions based on
the source and destination addresses inside.
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6. Characteristic
► It is designed to handle high-performance LAN traffic, so
Layer 3 switch can be placed anywhere within a network
core or backbone, easily and cost-effectively replacing the
traditional backbone router.
► The switches run routing protocols, such as open shortest
path first (OSPF) or routing information protocol (RIP), to
communicate with other Layer 3 switches or routers and to
build their routing/forwarding tables. These tables are
looked up to determine the route for an incoming packet
► a Layer 3 switch can reprogram the hardware dynamically
with the current Layer 3 routing information. This is what
allows much faster packet processing.
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7. Similarities
A Layer 3 switch does everything to a packet that a
traditional router does:
► Determines forwarding path based on Layer 3
information
► Validates the integrity of the Layer 3 header via
checksum
► Verifies packet expiration and updates accordingly
► Processes and responds to any option information
► Updates forwarding statistics in the Management
Information Base (MIB)
► Applies security controls if required
► Both types of boxes share a similar appearance
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8. Differences
► The key difference between Layer 3 switches and routers
lies in the hardware technology used to build the unit. The
hardware inside a Layer 3 switch merges that of traditional
switches and routers, replacing some of a router's software
logic with hardware to offer better performance in some
situations.
 In general-purpose routers, packet switching takes place
using a microprocessor, whereas a Layer 3 switch
performs this using application-specific integrated
circuit (ASIC) hardware.
 Another differentiating feature between a router and a
Layer 3 switch is the number of ports to which
individual terminal devices can be connected. A Layer 3
switch usually has a significantly greater port density.
► Layer 3 switches often cost less than traditional routers
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9. Functionality
► The principle of a Layer 3 is "route once, switch many".
 A Layer 3 switch can actually store MAC to IP
Address associations for the source and destination of
the packet.
 when there is a packet with a new source destination
pair, it is routed (which means that the routing table
is referred to for determining the best path ) which is
time consuming, and this source destination pair
information is stored in the router cache.
 when the next packet arrives with the same source
destination pair, no routing lookup is needed and the
packet is directly switched (which is much faster).
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10. The benefits of Layer 3 switching
The benefits of layer 3 switching include the following:
► Hardware-based packet forwarding
► High-performance packet switching
► High-speed scalability
► Low latency
 Latency is the time taken for a packet of data from the
sending application to the receiving application. This
includes both transit time over the network, and
computer processing time at both the source and
destination computers.
► Lower per-port cost
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11. Cont…
► Flow accounting
► Security
► Quality of service
 ( QoS) refers to control mechanisms that can provide
different priority to different users or data flows, or
guarantee a certain level of performance to a data flow
in accordance with requests from the application
program
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12. $$$ Cost $$$
► Cost Savings
A traditional router may run U.S. $8,000 to $10,000 per
Fast Ethernet interface, while a Layer 3 switch costs less
than U.S. $1,000 per port.
 Layer 3 switching is cheap because Layer 3 switches are
targeted specifically for inter-VLAN routing, where only
Ethernet access technologies are used in high densities
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13. What is VLAN
► A virtual LAN, is a method of creating independent
logical networks within a physical network. Several VLANs
can co-exist within such a network. This helps in reducing
the broadcast domain and aids in network administration
by separating logical segments of a LAN (like company
departments) that should not exchange data using a LAN
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15. Do Layer 3 Switches Completely
Eliminate Need for the Traditional
Router
► Routers are still needed, especially where connections to
the wide area are required. Layer 3 switches may still
connect to such routers to learn their tables and route
packets to them when these packets need to be sent over
the WAN.
► The switches will be very effective on the workgroup and
the backbone within an enterprise, but most likely will not
replace the router at the edge of the WAN .
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16. ► Routers perform numerous other functions like
 filtering with access lists,
 inter-Autonomous System (AS) routing with protocols
such as the Border Gateway Protocol (BGP)
Some Layer 3 switches may completely replace the need for
a router if they can provide all these functions
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17. The End
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