4: Bridges and Switches in Objective-C

Creator QR Code 2d barcode in Objective-C 4: Bridges and Switches

4: Bridges and Switches
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A collision domain basically includes all the devices that share a media type at layer 1 of the OSI Reference Model such as all the devices on a single strand of 10Base5 or 10Base2 cabling or all the devices connected to a hub or hubs In a collision domain, each device on the segment will experience the effects of the collisions The more devices on the segment, the more likely it is that collisions will create bandwidth problems for these devices This is not to say that collisions are bad it s just that collisions are part of how Ethernet functions
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Hubs do not solve collision problems since they replicate physical layer signals In other words, hubs don t create
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extra bandwidth for attached devices Instead, they extend cable distances and repeat/amplify physical layer signals
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Bridges were originally used to solve collision and bandwidth problems Each port connected to a bridge is a separate collision domain When a frame is pulled into a port on a bridge, the bridge checks the frame s field checksum sequence (FCS), and if the FCS if valid, the frame is forwarded out of a destination port or ports Recall from 3 that the FCS is a checksum to ensure that the destination can check that the Ethernet frame has not been corrupted Basically, the bridge is creating the illusion that all physical segments to which it is connected are actually one large logical segment All devices connected to this logical segment are in the same broadcast domain this makes sense because bridges flood broadcasts Note that if you are having problems with large amounts of broadcasts, bridges will not solve these problems
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Bridges are data link layer devices that switch frames between different layer 2 segments or cables They perform their switching in software, and their switching decisions are based on the destination MAC address in the header of the data link layer frame
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Bridges perform three main functions:
They learn where devices are located by placing the MAC address of a NIC
and the identifier of the bridge port to which it is connected in a port address table
They forward traffic intelligently, drawing on information they have in their
port address table
They remove layer 2 loops by running the Spanning Tree Protocol (STP)
Actually, these three functions are implemented in bridges that perform transparent bridging Other types of bridging include translational bridging, source route bridging, source route transparent bridging, and source route translational bridging However, this book focuses on transparent bridging
The three main functions of a bridge are to learn the MAC addresses and port locations of devices, intelligently
forward traf c to a speci c destination or ood broadcasts and multicasts, and remove layer 2 loops
Learning Function
One of the three functions of a bridge is to learn which devices are connected to which ports of the bridge The bridge then uses this information to switch frames intelligently When a bridge receives a frame, it reads the source MAC address in the frame and compares it to a local MAC address table, called a port address table If the address is not already in this table, the bridge adds the address and the port identifier or number on which the frame was received If the address is already in the table, the bridge resets the timer for the table entry Entries in the table remain there as long as the bridge sees traffic from them; otherwise, the bridge ages out the old entries to allow room for newer ones
Forwarding Function
The second function of a bridge is to forward traffic intelligently To do this, the bridge uses the port address table to help it find where destinations are located When a frame is received on a port, the bridge first performs its learning function
4: Bridges and Switches
and then performs its forwarding function The bridge examines the destination MAC address in the frame header and looks for a corresponding entry in the port address table If the bridge finds a matching entry, the bridge forwards the frame out of the specified port If the port is the same port on which the frame was received (the source and destination are connected to the same port), the bridge drops the frame If the bridge Remember that these doesn t find an entry, or if the destination MAC three types of traf c are always ooded: address is a broadcast or multicast address, the unknown unicast addresses, broadcasts, bridge floods the frame out all of the remaining and multicasts ports
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