How a Switch Works

How a Switch Works

1. “Ethernet LANs,”

   • The logical network address
   • Physical data-link address
     • (MAC address)
       • The physical address of the device and is 48 bits (6 bytes) long
         • It is made up of two parts:
           • the organizational unique identifier (OUI)
           • the vendor-assigned address,
       • MAC address is used for the Fast Ethernet adapter on the computer

2. Transmission Methods

   • LAN data transmissions at Layer 2 fall into three classifications:
     • Unicast
       • Is a one-to-one transmission method
     • Multicast
       • is a one-to-many transmission method in which the network carries a message to multiple receivers at the same time
     • Broadcast
       • is a one-to-all transmission method in which the network carries a message to all devices at the same time

3. Layer 2 Switching Methods

   • Store-and-Forward Switching
     • CRC is an error-checking method that uses a mathematical formula, based on the number of bits (1s) in the frame
   • Store-and-Forward Switching Operation
     • requires more memory and processor (central processing unit, CPU)
     • entire frame in internal memory and check the frame for errors
     • bad data frames are discarded rather than forwarded across the network,
   • Cut-Through Switching
     • switch copies into its memory only the destination MAC address
     • The switch looks up the destination MAC address in its switching table
     • outgoing interface port
     • inspects each received frame’s
       • before
         • the frame’s destination network segment
   • Cut-Through Switching Operation
     • switches do not perform any error checking of the frame because the switch looks only for the frame’s destination MAC address and forwards the frame out the appropriate switch port.

4. Layer 3 Switching

   • Layer 3 switching is another example of fragmentless switching. So far, this discussion has focused on switching and bridging the data link layer (Layer 2) of the Open System Interconnection (OSI) model. When bridge technology was developed for the first time, it was not practical to build cable speed bridges with large amounts of high-speed ports due to the manufacturing cost involved.
     • Layer 3 Switching Operation
       • A switch is a Layer 2 device (data link) with physical ports and that the the switch communicates through the frames that are placed on the cable in Layer 1 (physical).
       • A router is a Layer 3 (network) device that communicates with other routers with the use of packets, which in turn are encapsulated within frames.

5. ARP Mapping

   • Address Resolution Protocol (ARP) is a network layer protocol used in IP to convert IP addresses to MAC addresses. A network device that seeks to learn a MAC address transmits an ARP request to the network. The host of the network that has the IP address in the request responds with its MAC address (hardware). This is called ARP allocation, the assignment of a layer 3 address (network) to a layer 2 address (data link).
     • The packet is intended for the router itself: the IP destination address (the combined network and portion of the station) corresponds to one of the IP addresses of the router. In this case, the packet must be passed to the appropriate top layer entity within the router and not forwarded to any external port.
     • The packet is intended for a known host on the directly connected network: this is the most common situation encountered by a network router. The router determines the allocation of the ARP table and forwards the packet to the appropriate interface port to the local network.
     • The ARP assignment for the specified host is unknown: the router initiates a discovery procedure by sending an ARP request that determines the network assignment to the hardware address. Because this discovery procedure takes time, although measured in milliseconds, the router could discard the packet that resulted in the discovery procedure in the first place

6. Fragmentation

   • Each output port on a network device has an associated maximum transmission unit (MTU). Remember that earlier in this chapter, the MTU indicates the largest frame size (measured in bytes) that can be carried in the interface. The MTU is often a function of the network technology in use, such as Ethernet, Token Ring, or Point-to-Point Protocol (PPP). PPP is used with internet connections. If the forwarded frame is larger than the available space, as indicated by the MTU, the frame is fragmented into smaller pieces for transmission in the particular network.
   • Bridges can not fragment frames when forwarding between LANs of different MTU sizes because data link connections rarely have a mechanism to reassemble fragments in the receiver. The mechanism is in the implementation of the network layer, as with IP, which is able to overcome this limitation. Net
   • Fragmentation is similar to taking a photograph and cutting it into pieces so that each piece fits in envelopes of different sizes to send by mail. It is up to the sender to determine the size of the largest piece that can be sent, and it is up to the receiver to reassemble these pieces.
   • Network layer packets can be divided into smaller parts if necessary so that these packets can travel through a link with a smaller MTU.
   • Fragmentation is a mixed blessing; Although it provides the means of communication through different link technologies, the processing that accomplishes the fragmentation is significant and it could be a burden for each device to fragment and reassemble the data. In addition, parts for reassembly can be taken out of service and can be released by the switch or router.
   • As a rule, it is best to avoid fragmentation in your network if possible. It is more efficient for the sending station to send packets that do not require fragmentation at any point of the route to the destination, instead of sending large packets that require intermediate routers to perform the fragmentation.

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