© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. © The McGraw-Hill Companies, Inc. Extending LANs Asst. Prof. Chaiporn Jaikaeo, Ph.D. Computer Engineering Department Kasetsart University, Bangkok, Thailand Adapted from the notes by Lami Kaya and lecture slides from Behrouz A. Forouzan

2 LAN Distance Limitation Distance limitation is a fundamental part of LAN designs Hardware is designed to emit a fixed amount of energy If wiring is extended beyond the design limits Stations will not receive a sufficiently strong signal, and errors will occur

3 Fiber Modem Extension The simplest LAN extension mechanism Consists of an optical fiber and a pair of fiber modems used to connect a computer to a remote Ethernet

4 Repeaters Operate at Physical layer

5 Hubs A form of repeaters operating in star topology ABCDEF

6 Bridges A bridge is a mechanism that connects two LAN segments A bridge has filtering capability It examines the MAC destination address in a frame And does not forward the frame onto the other LAN segment unless necessary A broadcast frame is delivered to all LAN segments Hence, it operates at Data Link Layer

7 Collision Domains Bridges help split collision domain into smaller ones

8 Bridging Two LANs ABCD

9 Learning Bridges Learning bridges automatically learn MAC addresses over time

10 Learning Bridges A  D E  A B  C

11 Loop Problem

12 Spanning Tree Protocol (STP) Protocol used among bridges to eliminate loops STP consists of three steps: Root election Bridge with the lowest ID will become the root bridge Shortest path computation Each bridge computes a shortest path to the root bridge. Links included in the shortest paths of all bridges form the spanning tree Forwarding An interface that connects to the shortest path is enabled for forwarding packets An interface that does not lie on the shortest path is blocked

13 STP – Example

14 Switches An Ethernet switch, sometimes called a Layer 2 switch is an electronic device that resembles a hub Hub  Multiport repeater Switch  Multiport bridge A 48-port Ethernet Switch (source: Wikipedia)

15 Switches Similar to multiport bridges Also run Spanning-Tree Protocol

16 Virtual LANs Virtual LAN (VLAN) switches divide computers into separate broadcast domains A broadcast frame is sent to only all computers in the same VLAN

17 Layer-3 Connecting Devices

18 Routers Capable of connecting networks of different types Routers separate networks into different broadcast domains WAN interfacesLAN interfaces