Differences between Layer 2, 3, 4 Switching / Multilayer Switching / Layer 3 Routing

Differences between layer 2, 3, 4 Switching, Multilayer switching and Routing has been given below according to the function/operation of devices
Layer 2 Switching Devices that forward frames at Layer 2 involve the following functions:
  • MAC addresses are learned from the incoming frames’ source addresses.
  • A table of MAC addresses and their associated bridge and switch ports is built and maintained.
  • Broadcast and multicast frames are flooded out to all ports (except the one that received the frame).
  • Frames destined for unknown locations are flooded out to all ports (except the one that received the frame).
  • Bridges and switches communicate with each other using the Spanning Tree Protocol to eliminate bridging loops. 
A Layer 2 switch performs essentially the same function as a transparent bridge; however, a switch can have many ports and can perform hardware-based bridging. Frames are forwarded using specialized hardware, called application-specific integrated circuits (ASIC). This hardware gives switching great scalability, with wire-speed performance, low latency, low cost, and high port density.

As long as Layer 2 frames are being switched between two Layer 1 interfaces of the same media type, such as two Ethernet connections or an Ethernet connection and a Fast Ethernet connection, the frames do not have to be modified. However, if the two interfaces are different media, such as Ethernet and Token Ring or Ethernet and Fiber Distributed Data Interface (FDDI), the Layer 2 switch must translate the frame contents before sending out the Layer 1 interface.

Layer 2 switching is used primarily for workgroup connectivity and network segmentation. You can contain traffic between users and servers in a workgroup within the switch. In addition, the number of stations on a network segment can be reduced with a switch, minimizing the collision domain size.

One drawback to Layer 2 switching is that it cannot be scaled effectively. Switches must forward broadcast frames to all ports, causing large switched networks to become large broadcast domains.
In addition, Spanning Tree Protocol (STP) can have a slow convergence time when the switch topology changes. STP also can block certain switch ports, preventing data transfer.
Layer 3 Routing 

Devices involved in Layer 3 routing perform the following functions:
  • Packets are forwarded between networks based on Layer 3 addresses.
  • An optimal path is determined for a packet to take through a network to the next router.
  • Packet forwarding involves a table lookup of the destination network, the next-hop router address, and the router’s own outbound interface.
  • An optimal path can be chosen from among many possibilities.
  • Routers communicate with each other using routing protocols.
By nature, routers do not forward broadcast packets and forward only multicast packets to segments with multicast clients. This action provides control over broadcast propagation and offers network segmentation into areas of common Layer 3 addressing.
Logical addressing is possible on a network with routers because the Layer 3 (network
layer) address uniquely identifies a device only at the network layer of the OSI reference model. Actual frame forwarding occurs using the Layer 2, or data link, address of devices. Therefore, some method must exist to associate a device’s data link layer (MAC) address with its network layer (IP) address. A router must also have addresses from both layers assigned to each of its interfaces connected to a network. This assignment gives the router the functionality to support the logical network layer addresses assigned to the physical networks.

In addition, a router must examine each packet’s Layer 3 header before making a routing decision.
Layer 3 security and control can be implemented on any router interface using the source and destination addresses, protocol, or other Layer 3 attribute to make decisions on whether to limit or forward the packets.

Layer 3 routing is generally performed by microprocessor-based engines, which require CPU cycles to examine each packet’s network layer header. The routing table of optimal paths to Layer 3 networks can also be a large table of dynamic values, requiring a finite lookup delay. Although you can place a router anywhere in a network, the router can become a bottleneck because of a latency of packet examination and processing.
Layer 3 Switching 

Devices involved in Layer 3 switching perform the following functions:
  • Packets are forwarded at Layer 3, just as a router would do.
  • Packets are switched using specialized hardware, ASIC, for high speed and low latency.
  • Packets can be forwarded with security control and quality of service (QoS) using Layer 3 address information.
  • Layer 3 switches are designed to examine and forward packets in high-speed LAN environments. Whereas a router might impose a bottleneck to forwarding throughput, a Layer 3 switch can be placed anywhere in the network, with little or no performance penalty.
Layer 4 Switching 

Devices involved in Layer 4 switching perform the following functions:
  • Packets are forwarded using hardware switching, based on both Layer 3 addressing and Layer 4 application information. (Layer 2 addressing is also inherently used.)
  • Layer 4 protocol types (UDP or TCP, for example) in packet headers are examined.
  • Layer 4 segment headers are examined to determine application port numbers.
Switching at Layer 4 allows finer control over the movement of information. For example, traffic can be prioritized according to the source and destination port numbers, and QoS can be defined for end users. Therefore, video or voice data can be switched at a higher level of service, with more bandwidth availability than file transfer or HTTP traffic. Layer 4 port numbers for source and destination also can perform traffic accounting.
A Layer 4 switch also must allocate a large amount of memory to its forwarding tables. Layer 2 and Layer 3 devices have forwarding tables based on MAC and network addresses, making those tables only as large as the number of network devices. Layer 4 devices, however, must keep track of application protocols and conversations occurring in the network. Their forwarding tables become proportional to the number of network devices multiplied by the number of applications.
Multilayer Switching
Devices involved in MLS perform the following functions:
  • Packets are forwarded in hardware that combines Layer 2, Layer 3, and Layer 4 switching.
  •  Packets are forwarded at wire speed.  
  • The traditional Layer 3 routing function is provided using Cisco Express Forwarding (CEF),in which a database of routes to every destination network is maintained and distributed to switching ASICs for very high forwarding performance.
Cisco switches perform multilayer switching at Layer 3 and Layer 4. At Layer 3, the Catalyst family of switches caches traffic flows based on IP addresses. At Layer 4, traffic flows are cached based on source and destination addresses, in addition to source and destination ports. All switching is performed in hardware, providing equal performance at both Layer 3 and Layer 4 switching.

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