Interconnecting LANs Chapter 4 PDF

Chapter 4 Interconnecting the LANs 1  Chapter Outline Introduction The Hub The Network Bridge The Network Switch The Router Interconnecting the LANs with the Router Configuring the Network Interface – Auto-negotiation...

Chapter 4 Interconnecting the LANs 1  Chapter Outline Introduction The Hub The Network Bridge The Network Switch The Router Interconnecting the LANs with the Router Configuring the Network Interface – Auto-negotiation 2  Chapter Objectives Describe how a hub is used to interconnect LANs. Describe how a bridge is used to interconnect LANs Describe how a switch is used to interconnect LANs. Discuss the advantages of using a switch instead of a hub Describe the function of a router when used to interconnect LANs Describe the interface associated with a router Describe the function of a gateway in a computer network Describe the concept of a network segment Describe the concept of auto-negotiation 3  Introduction The utility of LANs led to the desire to connect two (or more) networks together. The objective of this chapter and subsequent chapters is to introduce the concepts and issues behind interconnecting LANs. Interconnecting LANs in a campus network or even interconnecting LANs in a wide area network (WAN) incorporate similar concepts and issues. The campus network is a collection of two or more interconnected LANs. Either can be within a building or housed externally in multiple buildings. 4  The Network Hub A hub is the most basic networking device connecting multiple computers or other network devices. A hub is a physical (layer 1) device. It’s a dumpy device (no intelligence). It works with 0’s and 1’s (Bits). It works with the broadcasting mechanism. Its bandwidths are shared by a number of connecting devices. 5 Hubs A hub is a physical layer networking device which is used to connect multiple devices in a network. They are generally used to connect computers in a LAN. ot.  The Network Bridge A bridge is used in computer networks to interconnect two LANs together and separate network segments. A segment is a section of a network separated by bridges, switches, and routers. The bridge is a layer 2 device in the OSI model, meaning that it uses the MAC address information to make decisions regarding forwarding data packets. Only the data that needs to be sent across the bridge to the adjacent network segment is forwarded. 7  The Network Bridge This figure shows an example of using a Port 1 Bridge Port 2 bridge to segment two Ethernet LANs. The Segment 2 Segment 1 picture shows that LAN A connects to port LAN A LAN B 1 of the bridge and LAN B connects to port Computer 1 Computer 4 Computer 2 Computer 5 2 on the bridge. This creates two segments Computer 3 Computer 6 as shown in the picture. There are three computers in LAN A and three computers Using a bridge to interconnect two Ethernet LANs. in LAN B. 8  Bridging Table The bridges use the MAC addresses to build a bridging table of MAC addresses and port Bridging Table locations for hosts connected to the bridge ports. MAC Address Port No. An example of the information contained in a 00-40-96-25-8E-AA (1) 1 bridging table is provided. 00-60-97-61-78-BB (2) 1 The table shows the stored MAC address and the 00-40-00-A5-85-CC (3) 1 port where the address was obtained. The source MAC address is stored into the bridge 00-3F-1E-25-85-DD (4) 2 table when a host talks (transmits a data packet) 00-40-75-A5-85-EE (5) 2 on the LAN. 00-40-96-25-85-FF (6) 2 9  The Network Bridge For example, if computer 1 in LAN A sends a message to computer 2 the bridge will store the MAC addresses of both computers and record that both of these computers are connected to port 1. 10  Address Resolution Protocol (ARP) All networking devices (e.g., computers) contain an ARP cache, temporary storage of MAC addresses recently contacted. This is also called the ARP table. The ARP cache holds the MAC address of a host and this enables the message to be sent directly to the destination MAC address without the computer having to issue an ARP request for MAC address. The ARP cache contents on a Windows computer can be viewed using the arp –a command while in the command prompt, as shown. 11  Address Resolution Protocol (ARP) This message is generated if all of the ARP entries have expired or deleted. 12 The Network Switch 13  Network Switch (layer 2) What is a switch Switches are networking devices operating at layer 2 or a data link layer of the OSI model. They connect devices in a network and use packet switching to send, receive or forward data packets or data frames over the network. A switch operates in the layer 2, i.e. data link layer of the OSI model. It is an intelligent network device that can be conceived as a multiport network bridge. It uses MAC addresses (addresses of medium access control sublayer) to send data packets to selected destination ports. It is supports unicast (one-to-one), multicast (one-to-many) and broadcast (one-to-all) communications. Switches are active devices, equipped with network software and network management capabilities. Types of Switches There are variety of switches that can be broadly categorised into 2 types − Unmanaged Switch − These are inexpensive switches commonly used in home networks and small businesses. They can be set up by simply plugging in to the network, after which they instantly start operating. Unmanaged switch Managed Switch − These are costly switches that are used in organisations with large and complex networks, since they can be customized to augment the functionalities of a standard switch. Managed switch 15  Network Switch Ports – the interface for the networking devices In this figure, the hub has been replaced with a switch. The change from a hub to a switch is relatively easy. The port connections are the same (RJ-45) and once the connections are changed and the device is powered on, the switch begins to make the direct data 16 connections for multiple ports using layer 2 switching.  Switch: Verifying the Link Light A networking connection can be verified by examining the link light on the hub. The presence of a link light indicates that the transmit and receive pairs are properly aligned and the connected devices are communicating. Absence of the link light Link Light indicates a possible cabling or hardware problem. 17  L2 Network Switch The LAN shown contains 14 computers and 2 printers connected to 16 ports on the switch. The LAN is configured in a star topology. If the computer connected to port 1 is printing a file on the laser printer (port 12), the switch will set up a direct connect between ports 1 and 12. The computer at port 14 could also be communicating with the computer at port 7 and the computer at port 4 could be printing a file on the color printer at port 16. The use of the switch enables simultaneous direct data connections for multiple pairs of hosts connected to the network. Each switch connection provides a link with minimal collisions and therefore maximum use of the LAN’s bandwidth. 18  L2 Network Switch A link with minimal collisions is possible since only the two computers that established the link will be communicating over the channel. Recall that in the star topology each host has a direct connection to the switch. Therefore, when the link is established between the two hosts, their link is isolated from any other data traffic. However, the exception to this is when broadcast or multicast messages are sent in the LAN. In the case of a broadcast message, the message is sent to all devices connected to the LAN. A multicast message is sent to a specific group of hosts on the network. 19  Switch Benefits The benefits of using a network switch are many in a modern computer network. These benefits include less network congestion, faster data transfers, excellent manageability. It has been shown that a network switch can replace the network hub and the advantage is that data traffic within a LAN is isolated. The term for this is isolating the collision domains which is breaking the network into segments. A segment is a portion of the network where the data traffic from one part of the network is isolated from the other networking devices. As a result, the LAN will exhibit faster data transfers and latency within the LAN will be significantly reduced. Reduced latency means that the data packets will arrive at the destination more quickly. , 20  Content Addressable Memory (CAM) Table Content-addressable memory (CAM) is a special type of computer memory used in certain very- high-speed searching applications. Switches learn the MAC addresses of the connected networking by extracting the MAC address information from the headers of Ethernet data packets. The switch will map the extracted MAC address to the port where the data packet came in. This information is stored in CAM – Content Addressable Memory. CAM is a table of MAC address and port mapping used by the switch to identify connected networking devices The MAC address and port information remain in CAM as long as the device connected to the switch port remains active. Switches limit the amount of time address and port information are stored in CAM. This is called aging time. 21  Static vs. Dynamic MAC Notice that the Dynamic Address tab is highlighted. This indicates that this is a listing of the MAC addresses that have been assigned dynamically. Dynamic assignment means that the MAC address was assigned to a port when a host was connected. There is also a tab for Static Addresses. A static MAC address indicates that the MAC address has been manually assigned to an interface and the port assignment does not expire. The Secure tab shows what switch ports have been secured. This means that a MAC address has been assigned to a port and the port will automatically disable itself if a device with a different MAC address connects to the secured. 22  MAC Aging Time The “Aging Time” is listed to be 300 seconds. Aging time is the length of time a MAC address remains assigned to a port. The assignment of the MAC address will be removed if there is no data activity within this time. If the computer with the assigned MAC address initiates new data activity, the aging time counter is restarted and the MAC address remains assigned to the port. The management window shows a switch setting for enabling “Aging”. This switch is used to turn off the aging counter so that a MAC address assignment on a port never expires. 23  Switching Modes There are two modes used in a switch to forward frames: store-and-forward and cut-through Store-and-Forward In this mode, the entire frame of data is received before any decision is made regarding forwarding the data packet to its destination. 24  Switching Modes Store-and –Forward The term switch latency is the length of time a data packet takes from the time it enters a switch until it exits. It’s measured from the time the first bit is received until it is transmitted. There is switch latency in this mode because the destination and source MAC addresses must be extracted from the packet and the entire packet must be received before it is sent to the destination. An advantage of the store-and-forward mode is the switch checks the data packet for errors before it is sent on to the destination. A disadvantage is lengthy data packets will take a longer time before they exit the switch and are sent to the destination. 25  Switching Modes Cut-Through In this mode, the data packet is forwarded to the destination as soon as the destination MAC address has been read. This minimizes the switch latency however, no error detection is provided by the switch. There are two forms of cut-through switching; Fast-Forward and Fragment Free. 26  Cut Through Fast Forward This mode offers the minimum switch latency as the received data packet is sent to the destination as soon as the destination MAC address is extracted. Fragment-Free In this mode, fragment collisions are filtered out by the switch. Fragment-collisions are collisions that occur within the first 64 bytes of the data packet. Recall from Chapter 1 that the minimum Ethernet data packet size is 64 bytes. The collisions create packets smaller than 64 bytes and these fragments are discarded. 27 Assembling an Office LAN  Assembling an Office LAN For this example, 3 computers and 1 printer are to be configured in the star topology. Each device in the network will be assigned an IP address from the private address space. Feature: Ethernet CAT5 twisted-pair cable Switch  Assembling, Configuring, and Testing the Office LAN Step 1 The first step in assembling an office LAN is to document the devices to be connected to the network and prepare a simple sketch of the proposed network. Each device’s MAC and IP addresses should be included in the network drawing documentation.  LAN Documentation Remember, each NIC contains a unique MAC address and the IP addresses are locally assigned by the network administrator. The MAC addresses were obtained by entering the ipconfig /all command from the command prompt in Windows XP. Document all IP addresses of the devices used in this office LAN. Note: In this class and this text, you will function as the network administrator. The network administrator must know how to obtain all IP and MAC address information for devices connected to the network. This requires that the network administrator keep good documentation of the network. The MAC and assigned IP address for the devices in the office LAN. Device Name MAC address IP address Computer 1 00-10-A4-13-99-2E 10.10.10.1 Computer 2 00-10-A4-13-6C-6E 10.10.10.2 Computer 3 00-B0-D0-25-BF-48 10.10.10.3 Laser Jet Printer 00-10-83-0B-A6-2F 10.10.10.20  Continue..... Step 2 Cable used to interconnect the networking devices. Connect all of the CAT5 CAT5e CAT6 networking devices together designing the star topology and RJ-45 – 8 pin modular connector use with CAT5, 5e, Twisted Pair Cable and 6 cable. Twisted Pair Cable shown in the picture  Configure the IP Addresses Step 3 Configure the IP address settings on each computer according to the list prepared by the network administrator Computer 1 00-10-A4-13-99-2E 10.10.10.1 Computer 2 00-10-A4-13-6C-6E 10.10.10.2 Computer 3 00-B0-D0-25-BF-48 10.10.10.3 Laser Jet Printer 00-10-83-0B-A6-2F 10.10.10.20 The Router 35  The Router The router is a layer 3 device in the OSI model, which means the router uses the network address (layer 3 addressing) to make routing decisions regarding forwarding the data packets. Remember, the OSI model separates the network responsibilities into different layers. In the OSI model, the layer 3 or Network layer responsibilities include handling the network address. The network address is also called a logical address rather than a physical address such as the MAC address. The logical address describes the IP address location of the network and the address location of the host in the network. 36  The Router Basically the router is configured to know how to route data packets entering or exiting the LAN. This differs from the bridge and the layer 2 switch which use the Ethernet address for making decisions regarding forwarding data packets and only know how to forward data to hosts physically connected to their ports. Routers are used to interconnect LANs in a campus network. Routers also make it possible to interconnect to LANs around the country and the world. The networks can use the same protocol (e.g. Ethernet) or they can be used to interconnect LANs that are using different layer 2 technologies such as a Ethernet and Token Ring. 37  Router Interface Cisco 2800 Series Router - Interface A Cisco 7200 series router interface. (Courtesy of Cisco Systems.) 38  Router Interface Console input—This input provides an RS-232 serial communications link into the router for initial router configuration. This router is also showing a USB input. Auxiliary input—This input is used to connect a dial-in modem into the router. The auxiliary port provides an alternate way to remotely log into the router if the network is down. This port also uses an RJ-45 connection. 39  Router Interface Serial Input - This is a serial connection and it has a built-in CSU/DSU. This interface is used to provide a T1 connection to the communications carrier. This type of connection (RJ-45) replaces the older cabling using V.35 cable 40  Router Interface FastEthernet Input – These are 100 Mbps interfaces for the router. Often the router ports are called the router interface, the physical connection where the router connects to the network. 41  Router Interface Voice Interface Card (VIC2-4FXO) – This interface shows 4 phone line connections. This router can be programmed as a small PBX (Private Branch Exchange) for use in a small office. 42  Router Interface Modem - This interface has two RJ-11 jacks and has two V.90 analog internal modems. These modems can be used to handle both incoming and outgoing modem calls. 43  Router Interface FXS/DID - This interface is a four-port FXS and DID voice/fax interface card. FXS is a Foreign Exchange Interface that connects directly to a standard telephone. DID is Direct Inward Dialing is a feature that enables callers to directly call an extension on a PBX. 44 Interconnecting LANs with the Router 45  Routers - Review A router routes data based on the destination network address or logical address rather than the physical address used by the layer 2 devices such as the switch and the bridge. Information exchanged with bridges and layer 2 switches requires that the MAC address for the hosts be known. Routed networks such as most enterprise and campus networks use IP addressing for managing the data movement. Enterprise network is a term used to describe the network used by a large company. The use of the network or logical address on computers allows the information to be sent from a LAN to a destination without requiring that the computer know the MAC address of the destination computer. Remember, final delivery of data packets is based on knowing the MAC address of the destination. 46 Router 47 Router Router A connects directly to the LAN A switch via FastEthernet port FA0/0. Router A also connects directly to Router B via the FastEthernet port FA0/1 and connects to Router C via FastEthernet port FA0/2. 48  Router Router B connects directly to the LAN B switch via FastEthernet port FA0/0. Router B also connects to the LAN C switch via FastEthernet port FA0/1. Router B connects directly to Router A via FastEthernet port FA0/2 and connects to Router C via FastEthernet port FA0/3. 49  Router Router C connects directly to the LAN D switch via the FastEthernet port FA0/0. Router C also connects to Router A via FastEthernet port FA0/2. Connection to Router B is provided via FastEthernet port FA0/1. 50  Serial Ports The serial ports (s0, s1, s2 …) are not being used to interconnect the routers in this example campus network. The serial interfaces are typically used to interconnect LANs that connect through a data communications carrier such as a telephone company or ISP. 51 Routing Table Delivery of the information over the network is made possible by the use of an IP address and routing tables. Routing tables keep track of the routes to used for forwarding data to its destination. 52 Gateway Address The term gateway is used to describe the address of the networking device that enables the hosts in a LAN to connect to networks and hosts outside the LAN. For all hosts in LAN A the gateway address will be 10.10.20.250. 53  Network Segment The network segment defines the networking link between two LANs. There is a segment associated with each connection of an internetworking device (e.g. router-hub, router-switch, router-router). For example, the IP address for the network segment connecting LAN A to the router is 10.10.20.0. All hosts connected to this segment must contain a 10.10.20.x since a subnet mask of 255.255.255.0 is being used. Subnet masking is fully explained in Chapter 5. 54