CCNA :) Be a Good Network Administrator

Summary This page summarizes the topics discussed in this module. Redundancy is defined as a duplication of components that allows continued functionality despite the failure of an individual component. In a network, redundancy means to have a backup method to connect all devices. Redundant topologies increase network reliability and decrease downtime caused by a single point of failure. A redundant switched topology may cause broadcast storms, multiple frame transmissions, and MAC address table instability problems. A broadcast storm is caused by multiple hosts that send and receive multiple broadcast messages. The result is that they continue to propagate broadcast traffic over and over until one of the switches is disconnected. During a broadcast storm, the network appears to be down or extremely slow. Multiple frame transmissions occur when a router receives multiple copies of a frame from multiple switches due to an unknown MAC address. These excessive transmissions cause the ...

Spanning-tree recalculation 7.2.6  This page will describe the convergence of a spanning-tree network. A switched internetwork has converged when all the switch and bridge ports are in either the forwarding or blocking state. Forwarding ports send and receive data traffic and BPDUs. Blocking ports only receive BPDUs. When the network topology changes, switches and bridges recompute the spanning-tree and cause a disruption in network traffic.   Convergence on a new spanning-tree topology that uses the IEEE 802.1d standard can take up to 50 seconds. This convergence is made up of the max-age of 20 seconds, plus the listening forward delay of 15 seconds, and the learning forward delay of 15 seconds. The Lab Activities will show students how to create and verify a basic switch configuration. The next page will introduce the Rapid Spanning-Tree Protocol. Rapid spanning-tree protocol 7.2.7  This page will describe the Rapid Span...

Selecting the root bridge 7.2.4  This page will explain how a root bridge is selected in an STP network. The first decision that all switches in the network make, is to identify the root bridge. The position of the root bridge in a network affects the traffic flow. When a switch is turned on, the spanning-tree algorithm is used to identify the root bridge. BPDUs are sent out with the bridge ID (BID). The BID consists of a bridge priority that defaults to 32768 and the switch MAC address. By default BPDUs are sent every two seconds. When a switch first starts up, it assumes it is the root switch and sends BPDUs that contain the switch MAC address in both the root and sender BID. These BPDUs are considered inferior because they are generated from the designated switch that has lost its link to the root bridge. The designated switch transmits the BPDUs with the information that it is the root bridge as well as the designated bridge. These BPDUs contain the switch MAC addr...

Spanning Tree Protocol 7.2.3 This page will teach students about the ports and devices that are found in an STP switched network. When the network has stabilized, it has converged and there is one spanning-tree per network. As a result, for every switched network the following elements exist: One root bridge per network One root port per non-root bridge One designated port per segment Unused, or non-designated ports Root ports and designated ports are used for forwarding (F) data traffic. Non-designated ports discard data traffic. These ports are called blocking (B) or discarding ports.  The next page will discuss the root bridge. 

Spanning Tree Protocol 7.2.2  This page will explain how STP can be used to create a loop free network. Ethernet bridges and switches can implement the IEEE 802.1d Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network.  Shortest path is based on cumulative link costs. Link costs are based on the speed of the link.  The Spanning-Tree Protocol establishes a root node called the root bridge. The Spanning-Tree Protocol constructs a topology that has one path for every node on the network. This tree originates from the root bridge. Redundant links that are not part of the shortest path tree are blocked. It is because certain paths are blocked that a loop free topology is possible. Data frames received on blocked links are dropped. The Spanning-Tree Protocol requires network devices to exchange messages to detect bridging loops. Links that will cause a loop are put into a blocking state. Switches send mes...

Spanning-Tree Protocol /  Redundant topology and spanning tree 7.2.1 This page will teach students how to create a loop free logical topology. Redundant network topologies are designed to ensure that networks continue to function in the presence of single points of failure. Work is interrupted less often for users because the network continues to function. Any interruptions that are caused by a failure should be as short as possible. Reliability is increased by redundancy. A network that is based on switches or bridges will introduce redundant links between those switches or bridges to overcome the failure of a single link. These connections introduce physical loops into the network. These bridging loops are created so if one link fails another can take over the function of forwarding traffic. When the destination of the traffic is unknown to a switch, it floods traffic out all ports except the port that received the traffic. Broadcasts and multicasts are also forwarded out...

Media access control database instability 7.1.6  This page will explain how incorrect information can be forwarded in a redundant switched network. In a redundant switched network it is possible for switches to learn the wrong information. A switch can incorrectly learn that a MAC address is on one port, when it is actually on a different port. In this example the MAC address of Router Y is not in the MAC address table of either switch. Host X sends a frame directed to Router Y. Switches A and B learn the MAC address of Host X on port 0. The frame to Router Y is flooded on port 1 of both switches. Switches A and B receive this information on port 1 and incorrectly learn the MAC address of Host X on port 1. When Router Y sends a frame to Host X, Switch A and Switch B also receive the frame and will send it out port 1. This is unnecessary, but the switches have incorrectly learned that Host X is on port 1. In this example the unicast frame from Router Y to Host X will...