McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Chapter 15 Connecting LANs, Backbone Networks, and Virtual LANs Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pongpisit Wuttidittachotti Faculty of Information Technology King Mongkut's University of Technology North Bangkok Line ID: pongpisitw

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Network Topologies A network’s geographical shape is referred to as the network’s topology. The five kinds of network topologies are: Bus topology Ring topology Star topology Hybrid topology Mesh topology

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Bus Topology The bus topology has a single cable, called the bus or the trunk, to which every device on the network connects. All of the messages on the bus pass by each device, or node, on the bus. It uses inexpensive coaxial cable that is easy to install. A break anywhere along the cable causes all network traffic to stop.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Ring Topology A ring topology forms when you connect a network’s nodes in a circle. Messages flow in a single direction. A small packet called a token circulates the ring – a device must have the token to use the network.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Star Topology In a star topology, each device in the network connects to a central hub, which distributes messages from one node to another. Provides centralized control Easy expansion Fault tolerance

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Hybrid Topology A hybrid topology in a network employs more than one topology to connect devices.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Mesh Topology In a mesh topology, multiple connections are made. In mission-critical operations, you want as much redundancy as possible so that if one part of the network goes down, the packets can find an alternate path to their destination.

McGraw-Hill©The McGraw-Hill Companies, Inc., Connecting Devices Five connecting devices Repeaters Hubs Bridges Switches Routers Gateway

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Figure 15.1 Five categories of connecting devices

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Repeaters A physical layer device the acts on bits not on frames or packets Can have two or more interfaces When a bit (0,1) arrives, the repeater receives it and regenerates it, the transmits it onto all other interfaces Used in LAN to connect cable segments and extend the maximum cable length  extending the geographical LAN range Ethernet 10base5 – Max. segment length 500m – 4 repeaters (5 segments) are used to extend the cable to 2500m) Ethernet 10Base2- Max. segment length 185m - 4 repeaters (5 segments) are used to extend the cable to 925m Repeaters do not implement any access method If any two nodes on any two connected segments transmit at the same time collision will happen

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Figure 15.3 Function of a repeater

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Figure 15.2 A repeater connecting two segments of a LAN

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Hubs Acts on the physical layer Operate on bits rather than frames Also called multiport repeater Used to connect stations adapters in a physical star topology but logically bus Connection to the hub consists of two pairs of twisted pair wire one for transmission and the other for receiving. Hub receives a bit from an adapter and sends it to all the other adapters without implementing any access method. does not do filtering (forward a frame into a specific destination or drop it) just it copy the received frame onto all other links The entire hub forms a single collision domain, and a single Broadcast domain Collision domain: is that part of the network (set of NICs) when two or more nodes transmit at the same time collision will happen. Broadcast domain: is that part of the network (set of NIC) where each NIC can 'see' other NICs' traffic broadcast messages. Multiple Hubs can be used to extend the network length For 10BaseT and 100BaseT the maximum length of the connection between an adapter and the hub is 100 meters  the maximum length between any two nodes is 200 m = maximum network length

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Ethernet

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Figure 16.4 Hubs

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Interconnecting with hubs Backbone hub interconnects LAN segments Advantage: Extends max distance between nodes Disadvantages Individual segment collision domains become one large collision domain  (reduce the performance) Can’t interconnect different Ethernet technologies(like 10BaseT & 100BaseT) because no buffering at the hub Here we have a single collision domain and a single broadcast domain

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Hubs Vs. Repeaters Hub are different than repeaters in the following: The provide network management features by gathering information about the network and report them to a monitoring host connected to the hub so some statistics about the network (bandwidth usages, collision rates, average frame sizes) can be generated. If an adapter is not working the hub can disconnect it internally and the network will not be affected.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Bridges/switches Acts on the data link layer (MAC address level) Used to divide (segment) the LAN into smaller LANs segments, or to connect LANs that use identical physical and data link layers protocol (see figure in next slide) Each LAN segment is a separate collision domain Bridge does not send the received frame to all other interfaces like hubs and repeaters, but it performs filtering which means: Whether a frame should be forwarded to another interface that leads to the destination or dropped This is done by a bridge table (forwarding table) that contains entries for the nodes on the LAN The bridge table is initially empty and filled automatically by learning from frames movements in the network An entry in the bridge table consists of : Node LAN (MAC) Address, Bridge Interface to which the node is connected to, the record creation time A bridge runs CSMA/CD before sending a frame onto the link not like the hub or repeater Bridge frame handling is done in software

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Bridges Connecting two or more LAN segments together

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Bridges (Switches) Vs. Hubs A Hub sending a packet form F to C. A Switch sending a packet from F to C

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Figure 15.5 A bridge connecting two LANs

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Legacy Ethernet HubHubSwitchSwitch

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Legacy Ethernet HubHub Half Duplex:Half Duplex: One way traffic.One way traffic. Necessary on a shared media.Necessary on a shared media. Only one device can transmit at a time.Only one device can transmit at a time. Collisions occur.Collisions occur.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 SwitchSwitch Legacy Ethernet Full Duplex:Full Duplex: Two way traffic.Two way traffic. Not a shared media.Not a shared media. Dedicated switch connection.Dedicated switch connection. A device can transmit and receive at the same time. A device can transmit and receive at the same time. No Collisions.No Collisions.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Ethernet Hubs and Switches

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Hubs and Switches Collision Domain: The area of a network where collisions can occur. Includes a hub and all connected devices. Each port on a switch is considered a separate collision domain even if there is only one device attached to the port. Broadcast Domain: The area of a network where connected devices can receive a broadcast. Usually includes Layer 1 and 2 devices. A collection of collision domains. A router (Layer 3 device) is the usual boundary since routers block broadcasts.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Legacy Ethernet: Using Hubs

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Legacy Ethernet: Using Hubs So, what does a hub do when it receives information?So, what does a hub do when it receives information? Remember, a hub is nothing more than a multiport repeater.Remember, a hub is nothing more than a multiport repeater.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Legacy Ethernet: Using Hubs The hub will flood it out all ports except for the incoming port. A hub is a Layer 1 device and does NOT look at Layer 2 addresses, so it is fast in transmitting data. A hub or series of hubs is called a single Collision Domain.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Legacy Ethernet: Using Hubs Disadvantages:Disadvantages: Collision DomainsCollision Domains Wasted BandwidthWasted Bandwidth All ports of the hub share the total available bandwidth.All ports of the hub share the total available bandwidth. Limited ScalabilityLimited Scalability Increased LatencyIncreased Latency Wasted Bandwidth

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Legacy Ethernet: Using Hubs Where is the collision domain?

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Legacy Ethernet: Using Hubs When a node wishes to communicate with ALL hosts on the network, it sends a BROADCAST frame with a destination MAC address of 0xFFFFFFFFFFFF. All nodes on the network recognize that they should look at the contents of the frame. Broadcasts are used in the Address Resolution Protocol (ARP), the Dynamic Host Configuration Protocol (DHCP) and other areas. (More Later!)

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Legacy Ethernet: Using Hubs Broadcasts cannot be entirely avoided if you are to have a dynamic network. However, too many broadcasts can create a lot of unnecessary traffic on a network and they should be minimized as much as possible. Layer 1 and Layer 2 devices (repeaters, hubs, bridges and switches) must forward a broadcast. Layer 1 devices have no choice because they do not look at MAC addresses. Layer 2 devices have no other choice because they cannot learn the 0xFFFFFFFFFFFF address.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Legacy Ethernet: Using Hubs Host 1111 sends a broadcast to all nodes on the network. All hosts recognize the MAC broadcast address and act on the information in the frame. Where is the Broadcast Domain?

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Ethernet: Using Switches

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Ethernet: Using Switches Switches are also known as learning bridges or learning switches. A switch has a source address table in cache (RAM) where it stores source MAC addresses for each port.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Ethernet: Using Switches Switch receives an Ethernet frame. Searches the source address table for the destination MAC address.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Ethernet: Using Switches If it finds a match, it forwards the frame by only sending it out that port (selective forwarding). If the destination address is not in the table, it floods it out all ports.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 How does a switch learn an address? First, the switch will see if the SA (1111) is in it’s table. If it is, it resets a timer. If it is NOT in the table it adds it, with the port number. Next the switch will flood the frame out all other ports, because the DA is not in the source address table.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 How does a switch learn an address? Most communications involve some sort of client-server relationship or exchange of information. Now 3333 sends data back to The switch sees if it has the SA stored. It does NOT so it adds it. Next, it checks the DA and sends it out port 1.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 How does a switch learn an address? Now, with both MAC addresses in the table, any information between 1111 and 3333 can be sent (selectively forwarded) out the appropriate port.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Multiple Transmissions - No Collisions Unlike a hub, a collision does NOT occur, which would cause the two PCs to have to retransmit the frames. The switch buffers the frames and sends them out port #6 one at a time. The sending PCs have no idea that there was another PC wanting to send to the same destination.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Collision Domains When there is only one device on a switch port, the collision domain is only between the PC and the switch. With a full-duplex PC and switch port, there will be no collision, since the devices and the medium can send and receive at the same time.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Switches: Five Basic Operations Learning: Adds source MAC address/port to the table. Aging: Addresses will be cleared after a specific length of inactivity. Flooding: Sends a frame out all ports if the SA is not in the table. Forwarding: Sends a frame out the proper port on a match. Filtering: Will not forward a frame out the received port. Drops corrupt frames.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Switch learning process When the switch receives a frame, it compares the source address of the frame with each entry in the forwarding table If No match is found, the bridge will add to the table the frame source address and the Interface on which the frame was received. If a match is found, the bridge updates the Interface number on which the frame was received if it is different from the one in the table also it updates the record time Then, the switch compares the destination address of the frame with each entry in the forwarding table (MAC table) If a match is found then The bridge compares the interface number on which the frame was received and the interface number in the table, if they are different the bridge forwards the frame through the interface number stored in the table. Otherwise, if they are the same the switches discards (drops) the frame. If no match is found, the switch floods the frame on all interfaces except the one on which the frame was received.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Some switch features Implements CSMA/CD switches Isolates collision domains (each LAN segment is a separate collision domain), THIS WILL REDUCE THE POSSIBILITY OF COLLISIONS AND result in higher total max throughput (see next slide) switch forwards a frame with broadcast address to all devices attached to the whole network (single broadcast domain) Can be used to combine Ethernet segments using different Ethernet technologies (10Base2 and 100BaseT and 10BaseT) because it has buffering capabilities Increases reliability (how?), performance (how?), and security (how?) Increases geographical coverage No limit on the size of the LANs connected through switches Transparent: installing or removing a switch does not require the stations networking software to be reconfigured. (“plug-and-play”): no configuration necessary at installation of switch /switch or when a host is removed from one of the LAN segments Disadvantage: switch does not allow multiple paths between LAN segments or between any two devices.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Figure Sharing bandwidth

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Collision domains in a nonbridged and bridged network In heavy load, each station has an average effective theoretical bandwidth = 10/12 Each station has an average effective bandwidth equal =10/3

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Example: Three LANs connected through a bridge Note: here we have three collision domains and a single broadcast domain Switch

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Figure 16.8 Prior to spanning tree application When using switches, the network should not contain any loop (there should be exactly one path from any LAN to any other LAN Loops can cause number of frames in the LAN to increase indefinitely Switch

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Effect of Loop of switches

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Figure 15.7 Loop problem in a learning switch

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Figure Forwarding and blocking ports after using spanning tree algorithm  For any connected graph there is a spanning tree that maintains connectivity but contains no closed loops  Loops are logically disabled by the minimum spanning tree algorithm

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Switches  N-Port bridge where N is equal to number of stations  Usually used to connect individual computers not LANs like bridge Allows more than one device connected to the switch directly to transmit simultaneously Can operates in Full-duplex mode (can send and receive frames at the same time over the same interface) Performs MAC address recognition and frame forwarding in hardware (bridge in software) Two types : Store-and-forward: switch receives the whole a frame on the input line, buffers it briefly, performs error checking, then routes it to the appropriate output line (similar to bridge). Buffering will cause some delay. Cut-through: based on the fact that the destination address appears at the beginning of the MAC frame, so once the address is recognized the frame is directly sent to the appropriate output line if the output buffer is empty (no need to buffer it).  no buffering delay  NO ERROR CHECKING

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Full-Duplex operation Isolated collision domains

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Routers Operates at network layer = deals with packets not frames Connect LANs and WANs with similar or different protocols together Switches and bridges isolate collision domains but forward broadcast messages to all LANs connected to them. Routers isolate both collision domains and broadcast domains Acts like normal stations on a network, but have more than one network address (an address to each connected network) Deals with global address ( network layer address (IP)) not local address (MAC address) Routers Communicate with each other and exchange routing information Determine best route using routing algorithm by special software installed on them Forward traffic if information on destination is available otherwise discard it (not like a switch or bridge)

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Figure Routers connecting independent LANs and WANs Routers

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 An Institutional Network Using Hubs, Ethernet Switches, and a Router

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 switch

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004

McGraw-Hill©The McGraw-Hill Companies, Inc., Virtual LANs If we want to move computers from group1 to group3, then rewiring (physical replacement) has to be done What is the alternative solution?? VLAN: Virtual (logical) Local Area Network : Local Area Network configured by software not by physical wiring 3 Collision domains 3 Broadcast domains

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Figure A switch using VLAN software  Using the Virtual LAN technology will allow grouping computers logically instead of physically.  VLAN divides the physical LAN into several Logical LANs called VLANs  Switch maintains a look up table to know to which LAN a machine belongs to. VLAN1: Ports 1,2,5,7 VLAN2: Ports 3,4,6 VLAN3: Ports 8,9, Separate broadcast domain  separate network

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Figure Two switches in a backbone using VLAN software

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 VLANs create broadcast domains. Note:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Advantages Of VLAN Reduce cost and installation time : Instead of physically moving a station to another segment or another switch, it can be moved by software. Increase security: A group of users needing a high security can be put into a VLAN so that NO users outside the VLAN can communicate with them. Stations belong to the same group can send broadcast messages that will NOT be received by users in others VLAN groups Creating Virtual Workgroups Stations located at physically different locations can be added easily to the same broadcast domain so that they can send broadcast messages to one another. EXAMPLE: people from different departments working on the same project