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 base 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 “inferior” BPDUs. These BPDUs contain the switch MAC address in both the root and sender BID. All switches see the BIDs sent. As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out. All bridges see these and decide that the bridge with the smallest BID value will be the root bridge. A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default, which will make the BID smaller. This should only be implemented when the traffic flow on the network is well understood. Lab Activity Lab Exercise: Selecting the Root Bridge In this lab, the student will create a basic switch configuration and verify it and determine which switch is selected as root switch with factory default settings. Lab Activity e-Lab Activity: Selecting the Root Bridge In this lab, the following functions will be performed. Verify configuration of hosts and switch by testing connectivity. Web Links Understanding Spanning-Tree Protocol http://www.cisco.com/univercd/cc/
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Content 7.2 Spanning-Tree Protocol 7.2.5 Stages of spanning-tree port states Time is required for protocol information to propagate throughout a switched network. Topology changes in one part of a network are not instantly known in other parts of the network. There is propagation delay. A switch should not change a port state from inactive to active immediately, as this may cause data loops. Each port on a switch that is using the Spanning-Tree Protocol has one of five states, as shown in Figure . In the blocking state, ports can only receive BPDUs. Data frames are discarded and no addresses can be learned. It may take up to 20 seconds to change from this state. Ports go from the blocked state to the listening state. In this state, switches determine if there are any other paths to the root bridge. The path that is not the least cost path to the root bridge goes back to the blocked state. The listening period is called the forward delay and lasts for 15 seconds. In the listening state, user data is not being forwarded and MAC addresses are not being learned. BPDUs are still processed. Ports transition from the listening to the learning state. In this state user data is not forwarded, but MAC addresses are learned from any traffic that is seen. The learning state lasts for 15 seconds and is also called the forward delay. BPDUs are still processed. A port goes from the learning state to the forwarding state. In this state user data is forwarded and MAC addresses continue to be learned. BPDUs are still processed. A port can be in a disabled state. This disabled state can occur when an administrator shuts down the port or the port fails. The time values given for each state are the default values. These values have been calculated on an assumption that there will be a maximum of seven switches in any branch of the spanning tree from the root bridge. Interactive Media Activity Point and Click: Spanning-Tree States When the student has completed this activity, the student will be able to identify the function of spanning-tree states. Web Links Understanding Spanning-Tree Protocol http://www.cisco.com/univercd/cc/ td/doc/product/ rtrmgmt/sw_ntman/ cwsimain/cwsi2/cwsiug2/ vlan2/stpapp.htm
Content 7.2 Spanning-Tree Protocol 7.2.6 Spanning-tree recalculation A switched internetwork has converged when all the switch and bridge ports are in either the forwarding or blocked state. Forwarding ports send and receive data traffic and BPDUs. Blocked ports will only receive BPDUs. When the network topology changes, switches and bridges recompute the Spanning Tree and cause a disruption of user traffic. Convergence on a new spanning-tree topology using 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. Lab Activity Lab Exercise: Spanning-Tree Recalculation In this lab, the student will create a basic switch configuration and verify it and observe the behavior of spanning tree algorithm in presence of switched network topology changes. Lab Activity e-Lab Activity: Spanning-Tree Recalculation In this lab, the students will create a basic switch configuration and verify it. Web Links Understanding Spanning-Tree Protocol http://www.cisco.com/univercd/cc/ td/doc/product/ rtrmgmt/sw_ntman/ cwsimain/cwsi2/ cwsiug2/vlan2/stpapp.htm
Content 7.2 Spanning-tree Protocol 7.2.7 Rapid Spanning-Tree Protocol The Rapid Spanning-Tree Protocol is defined in the IEEE 802.1w LAN standard. The standard and protocol introduce the following: The “blocked” state of a port has been renamed as the “discarding” state. A role of a discarding port is an “alternate port”. The discarding port can become the “designated port” in the event of the failure of the designated port for the segment. Link types have been defined as point-to-point, edge-type, and shared. These changes allow failure of links in switched network to be learned rapidly. Point-to-point links and edge-type links can go to the forwarding state immediately. Network convergence does not need to be any longer than 15 seconds with these changes. The Rapid Spanning-Tree Protocol, IEEE 802.1w, will eventually replace the Spanning-Tree Protocol, IEEE 802.1D. Web Links Understanding Rapid Spanning-Tree Protocol (802.1w) http://www.cisco.com/warp/
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Content Summary An understanding of the following key points should have been achieved: