Notes On Chapter Seventeen -- LAN Extensions: Fiber Modems, Repeaters, Bridges and Switches

• 17.0 Study Guide
  
  
  • Be able to explain what a repeater does and what a bridge does, and the difference between them.
  • Be able to explain how bridges learn which frames to forward and which frames not to forward. Be able to explain how the learning relates to source addresses in frames, and how the forwarding relates to destination addresses in frames.
  • Be able to explain why it is better to join network segments with bridges then to join them with repeaters - the advantage of simultaneous communication on different segments.
  • Be able to explain what an Ethernet hub does, and what an Ethernet switch does and the difference between them.
  • Be able to explain what a VLAN Ethernet switch does, and the difference between a VLAN Ethernet switch and an ordinary Ethernet switch.
  
  
• 17.1 Introduction
  
  
  • Mechanisms that can extend a LAN
  • LAN Switching
  • Repeaters and Bridges
  • The Spanning Tree Algorithm
  
  
• 17.2 Distance Limitation and LAN Design
  
  
  • LAN's are designed to operate within a given cost, and within certain power output, capacity and delay limits.
  • If LAN wiring is extended beyond a certain point, attenuation and/or excessive delay will cause errors.
  
  
• 17.3 Fiber Modem Extensions
  
  
  • An Ethernet can be extended with a segment of fiber with a pair of fiber modems at each end.
  • Delay and attenuation along the fiber is very low, so the extension can be much longer than if it were made out of wire cabling.
  
  
  
  
• 17.4 Repeaters
  
  
  • A repeater is a simple analog device that inputs a signal and outputs the exact same signal, except amplified.
  • Repeaters do not understand packets or signal coding.
  • Repeaters were much used with the original Ethernet.
  • Nowadays repeaters used with infrared networks is fairly common.
  
  
  
  
• 17.5 Bridges and Bridging
  
  
  • A bridge is a device used to connect two LAN segments.
  • Two segments of length X and Y, joined by a bridge will work in a situation where one segment of length X+Y would exceed the operating limitations of the LAN.
  • Bridges are higher-level devices than repeaters.
  • Unlike a repeater, a bridge does understand network frames (packets).
  • Each connection between a bridge and a network is an NIC.
  • A bridge examines all frames arriving at either of its interfaces, and forwards packets from each side to the other.
  • Unlike a repeater, a bridge does not forward parts of a frame before receiving the entire frame. It receives an entire frame, examines the frame, and then forwards it.
  • When a frame is transmitted by a bridge, it is transmitted at the same full power the original sender would have used.
  
  
  
  
• 17.6 Learning Bridges and Frame Filtering
  
  
  • When a packet arrives at the interface on one side of a bridge, the bridge may not forward it.
  • Bridges learn, and after learning, they can tell whether packets actually need to be forwarded.
  • How a bridge learns: it looks at the (MAC) source addresses of each packet that arrives at either of its network interfaces. Say it has interface 1 and interface 2. If a packet arrives at, say, interface 1, the bridge learns that the sender is on the same side of the bridge as interface 1.
  • The bridge makes two lists of addresses: Addresses of NIC's known to be on "side 1" and NIC's known to be on "side 2"
  • Bridges also look at the MAC destination addresses in arriving packets.
  • Suppose a packet arrives at interface 2 of a bridge, and the destination address is that of an NIC that bridge knows is on the same side as interface 2. Then the bridge will not forward that packet.
  • There's no need to forward such packets. The sender and receiver are both on the same side of the bridge.
  • If the destination address of a packet is the broadcast address (all one's) or a multicast address, the bridge forwards the packet.
  
  
  
  
• 17.7 Why Bridging Works Well
  
  
  • Because a bridge forwards selectively, it is possible for a pair of computers on one side of a bridge to communicate simultaneously as another pair on the other side of the bridge communicates.
  • This is important - a bridged network can support much more "communication per second" than a network that is not bridged.
  • DSL and cable modems function as selective bridges between the subscriber's local network and the ISP network.
  
  
• 17.8 Distributed Spanning Tree
  
  
  • If cycles exist in a bridged LAN, the bridges communicate with each other and mutually agree only to forward packets along a "spanning tree" subset of the network.
  • If this were not done, there could be problems such as broadcasts being forwarded around and around the network forever, or learning bridges becoming confused when trying to determine on which 'side' sending hosts reside.
  
  
  
  
• 17.9 Switching and Layer 2 Switches
  
  
  • An Ethernet switch is different from an Ethernet hub.
  • When hosts connect to a switch, the effect is as if each host is placed on an Ethernet segment all by itself, and there are bridges connecting each segment to all the other segments.
  • If a switch has N ports, it is possible for N/2 mutually disjoint pairs of computers to communicate through the switch simultaneously.
  • In effect, a hub emulates an Ethernet segment that is shared by all the hosts that connect to the hub.
  • No matter how many ports a hub has, only one pair of computers at a time can communicate through it.
  
  
  
  
• 17.10 VLAN Switches
  
  
  • An ISP or other organization may configure Virtual Local Area Network switches (VLAN switches) to emulate multiple logically independent switched networks.
  
  
• 17.11 Multiple Switches and Shared VLANS
  
  
  • It is common to connect computers in a small locality to a switch, and to connect switches to form a single network.
  • In such multiple-switch environments, for technical reasons, VLANs require special support.
  • A common solution is an IEEE standard, 802.1Q, that adds a 4-byte VLAN tag to Ethernet frames. When a frame crosses from one switch to another, the tag identifies the VLAN to which the frame belongs.
    
    
    
    
  • The tags are used only when frames travel directly from one switch to another switch.
    
    
    
    
  
  
• 17.12 The Importance of Bridging
  
  
  • Standalone bridges are not much-used in LANs nowadays.
  • However bridge technology remains fundamental and is incorporated in many LAN devices such as DSL and cable modems.
  • Many wireless routers have several ports that function as an Ethernet switch, which simulates a set of bridged segments.