Content Overview Redundancy in a network is extremely important because redundancy allows networks to be fault tolerant. Redundant topologies protect against network downtime due to a failure of a single link, port, or networking device. Network engineers are often required to make difficult decisions, balancing the cost of redundancy with the need for network availability. Redundant topologies based on switches and bridges are susceptible to broadcast storms, multiple frame transmissions, and MAC address database instability. Therefore network redundancy requires careful planning and monitoring to function properly. Switched networks provide the benefits of smaller collision domains, microsegmentation, and full duplex operation. Switched networks provide better performance. Redundancy in a network is required to protect against loss of connectivity due to the failure of an individual component. Providing this redundancy, however, often results in physical topologies with loops. Physical layer loops can cause serious problems in switched networks. Broadcast storms, multiple frame transmissions, and media access control database instability can make such networks unusable. The Spanning-Tree Protocol is used in switched networks to create a loop free logical topology from a physical topology that has loops. Links, ports, and switches that are not part of the active loop free topology do not participate in the forwarding of data frames. The Spanning-Tree Protocol is a powerful tool that gives network administrators the security of a redundant topology without the risk of problems caused by switching loops. Students completing this module should be able to:
Content 7.1 Redundant Topologies 7.1.1 Redundancy Many companies and organizations increasingly rely on computer networks for their operations. Access to file servers, databases, the Internet, intranets, and extranets is critical for successful businesses. If the network is down, productivity is lost and customers are dissatisfied.Companies are increasingly looking for 24 hour, seven day a week uptime for their computer networks. Achieving 100% uptime is perhaps impossible but securing a 99.999% or five nines uptime is a goal that organizations set. This is interpreted to mean one day of downtime, on average, for every 30 years, or one hour of downtime, on average, for every 4000 days, or 5.25 minutes of downtime per year. Achieving such a goal requires extremely reliable networks. Reliability in networks is achieved by reliable equipment and by designing networks that are tolerant to failures and faults. The network is designed to reconverge rapidly so that the fault is bypassed. Fault tolerance is achieved by redundancy. Redundancy means to be in excess or exceeding what is usual and natural. How does redundancy help achieve reliability? Assume that the only way to get to work is by a car. If the car develops a fault that makes it unusable, going to work will be impossible until it is repaired and returned. If the car fails and is unavailable, on average one day in ten then there is 90% usage. Going to work is possible nine days in every ten. Reliability is therefore 90%. Buying another car will improve matters. There is no need for two cars just to get to work. However, it does provide redundancy (backup) in case the primary vehicle fails. The ability to get to work is no longer dependent on a single car. Both cars may become unusable simultaneously, one day in every 100. Purchasing a second redundant car has improved reliability to 99%. Web Links Redundancy and Broadcast Storms http://www.howstuffworks.com/ lan-switch5.htm
Content 7.1 Redundant Topologies 7.1.2 Redundant topologies A goal of redundant topologies is to eliminate network outages caused by a single point of failure. All networks need redundancy for enhanced reliability. A network of roads is a global example of a redundant topology. If one road is closed for repair there is likely an alternate route to the destination. Consider an outlying community separated by a river from the town center. If there is only one bridge across the river there is only one way into town. The topology has no redundancy. If the bridge is flooded or damaged by an accident, travel to the town center across the bridge is impossible. Building a second bridge across the river creates a redundant topology. The suburb is not cut off from the town center if one bridge is impassable. Web Links Understanding Spanning-Tree Protocol – the Fundamental Bridging Algorithm http://www.oreillynet.com/pub/a/ network/2001/03/30/ net_2nd_lang.html
Content 7.1 Redundant Topologies 7.1.3 Redundant switched topologies Networks with redundant paths and devices allow for more network uptime. Redundant topologies eliminate single points of failure. If a path or device fails, the redundant path or device can take over the tasks of the failed path or device. If Switch A fails, traffic can still flow from Segment 2 to Segment 1 and to the router through Switch B. If port 1 fails on Switch A then traffic can still flow through port 1 on Switch B. Switches learn the MAC addresses of devices on their ports so that data can be properly forwarded to the destination. Switches will flood frames for unknown destinations until they learn the MAC addresses of the devices. Broadcasts and multicasts are also flooded. A redundant switched topology may cause broadcast storms, multiple frame copies, and MAC address table instability problems. Web Links Understanding Spanning-Tree Protocol – the Fundamental Bridging Algorithm http://www.oreillynet.com/pub/a/ network/2001/03/30/ net_2nd_lang.html
Content 7.1 Redundant Topologies 7.1.4 Broadcast storms Broadcasts and multicasts can cause problems in a switched network. Multicasts are treated as broadcasts by the switches. Broadcasts and multicasts frames are flooded out all ports, except the one on which the frame was received. If Host X sends a broadcast, like an ARP request for the Layer 2 address of the router, then Switch A will forward the broadcast out all ports. Switch B, being on the same segment, also forwards all broadcasts. Switch B sees all the broadcasts that Switch A forwarded and Switch A sees all the broadcasts that Switch B forwarded. Switch A sees the broadcasts and forwards them. Switch B sees the broadcasts and forwards them. The switches continue to propagate broadcast traffic over and over. This is called a broadcast storm. This broadcast storm will continue until one of the switches is disconnected. The switches and end devices will be so busy processing the broadcasts that user traffic is unlikely to flow. The network will appear to be down or extremely slow. Web Links Shining the Lights on Broadcast Storms http://www.networkuptime.com/tips/lights/
Content 7.1 Redundant Topologies 7.1.5 Multiple frame transmissions In a redundant switched network it is possible for an end device to receive multiple frames. Assume that the MAC address of Router Y has been timed out by both switches. Also assume that Host X still has the MAC address of Router Y in its ARP cache and sends a unicast frame to Router Y. The router receives the frame because it is on the same segment as Host X. Switch A does not have the MAC address