Layer 3 Switches

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    Layer 3 Switches
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Topics Covered

• Definitions
• Characteristics
• Comparison between Routers and Layer 3 Switches
• Functionality
• The Benefits of Layer 3 Switching
• Cost

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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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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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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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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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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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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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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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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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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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Cost

• Cost SavingsA 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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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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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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• 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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• The End