1. McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Chapter 16 Connecting LANs, Backbone Networks, and Virtual LANs

2. McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Connecting devices A repeater connects segments of a LAN. A repeater forwards every frame; it has no filtering capability. A repeater is a regenerator, not an amplifier. Amplifier cannot discriminate between the intended signal and noise; it amplifies equally everything fed into it. A repeater does not actually connect two LANs; it connects two segments of the same LAN. A repeater is not a device that can connect two LANs of different protocols.

3. McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Repeater Repeater extends length and thus overcomes the length restriction of 10Base5 An amplifier cannot discriminate between the intended signal and noise; it amplifies equally everything fed into it. A repeater does not amplify the signal; it regenerates the signal.

4. McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Function of a Repeater When a repeater receives a weakened or corrupted signal, it creates a copy, bit for bit, at the original strength. Repeater must be places so that a signal reaches it before any noise changes the meaning of any of its bits.

5. McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Hubs Hub is actually a multi-port repeater. Normally used to create connections between stations in a physical star topology. Removes the length restriction of 10Base-T

6. McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Operates in both the physical and data link layers As physical layer device, it regenerates the signal it receives. As a data link layer device, the bridge can check the MAC (source and destination) address contained in the frame. A bridge has a table used in filtering decisions. Difference in functionality between a bridge and a repeater? A bridge has filtering capability. It can check the destination address of a frame and decide if the frame should be forwarded or dropped. If the frame is to be forwarded, the decision must specify the port. A bridge has a table that maps addresses to ports. A bridge does not change the physical (MAC) addresses in a frame. Bridge

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

8. McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Learning Bridge Transparent bridge is a bridge in which the stations are completely unaware of the bridge’s existence. Frames must be forwarded from one station to another. Forwarding table is automatically made by learning frame movements in the network. Otherwise, administrator should do static entries in the table. Loops in the system must be prevented.

9. McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Loop Problem A frame sent out by bridge 1 is received by bridge 2, which has no idea about the destination. Bridge 2’s copy is received by bridge 1 and sent out due to lack of information about destination. Each frame is handled separately by using CSMA/CD.

10. McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Prior to Spanning Tree Application A bridge with smallest ID is selected as root bridge Mark one port of each bridge (except for the root bridge) as the root port. A root port is the port with the least-cost path from the bridge to the root bridge. Choose a designated bridge for each LAN. A designated bridge has the least-cost path between the LAN and the root bridge. Make the corresponding port the designated port. If two bridges have the same least-cost value, choose the one with the smaller ID. Make the root port and designated port as forwarding ports, the others are blocking ports.

11. McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Applying Spanning Tree B1 has the least ID and so it is the root bridge. Root ports are marked with one star. Designated bridges have an arrow pointing to them from the corresponding LAN. Designated ports are marked by two stars.

12. McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Forwarding Ports and Blocking Ports Blocking port is shown with a broken line. No loop exists.

13. McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Source Routing Bridges Source Routing bridge A sending station defines the bridges that the frame must visit. The addresses of these bridges are included in the frame. These addresses are obtained through special exchange frames Source routers are used with Token Ring LANs.

14. McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Bridges connecting Different LANs There are many issues: (Ethernet, ATM, Wireless) Frame Format: Each LAN has its own frame format. Maximum Data Size: Bridges must discard any frames too large for its system. Data Rate: The bridge must buffer the frame to compensate for the difference. Bit Order: Each LAN has its own strategy in sending the bits; MSB or LSB. Security: Wireless LANs implement security measures in the data link layer, however, Ethernet don’t. Bridge must encrypt a data received from wireless before forwarding to Ethernet LAN. Multimedia Support: some LANs support multimedia and QoS needed for this type of communication; others do not.

15. McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Bus Backbone A backbone network allows several LANs to be connected. In a backbone network, no station is directly connected to the backbone; the stations are part of a LAN, and the backbone connects the LANs. In a bus backbone, the topology of the backbone is a bus. Ex.: A bus backbone is one that connects single or multiple floor buildings on a campus. Only needed traffic is passed over from one bridge to another.

16. McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Star Backbone Collapsed or switched backbone In a star backbone, the topology of the backbone is a star; the backbone is just one switch Used as a distribution backbone inside a building. We often find a rack or chassis in the basement where the backbone switch and all hubs or switches are installed.

17. McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Connecting Remote LANs This type of backbone network is useful when a company has several offices with LANs and needs to connect them. The connection can be done through bridges, sometimes called remote bridges. Point-to-point network in this case is considered a LAN without stations.

18. McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 A switch connecting three LANs LAN setup is physical and geographic. Virtual LAN as a local area network configured by software, not by physical wiring.

19. McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 A switch using VLAN software VLAN technology is to divide a LAN into logical, instead of physical, segments. A LAN can be divided into several logical LANs called VLANs. Each VLAN is a work-group in the organization. If a person moves from one group to another, there is no need to change the physical configuration.

20. McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Two switches in a backbone using VLAN software VLANs create broadcast domains All members belonging to a VLAN can receive broadcast messages sent to that particular VLAN. Membership in VLAN can be based on physical port numbers or MAC addresses or IP addresses or Multicast IP addresses or in combinations.

21. McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Configuration of VLANs Manual configuration Automatic configuration Semiautomatic configuration Communication between switches Using frame tagging (When a frame is travelling between switches, an extra header is added to the MAC frame to define the destination VLAN) 802.1Q standard. Uses Time-Division Multiplexing Advantages of VLANs Cost and Time reduction in configuring. Creating virtual workgroups Security: Any broadcast message sent by a group members is received only by the specific group members.