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ccna exam dump free download.S2 (VLAN200) J V V J V JChapter 14 STP and Switch StackingTo reduce network downtime, redundant topologies are often used in network design, and redundancy is a key design to maintain network reliability. Multiple physical links between devices can provide redundant paths that allow the network to continue to operate when a single link or port fails, while redundant links can increase network capacity and provide traffic load sharing. To avoid creating Layer 2 switching loops, Layer 2 redundancy can be managed through STP, which allows a network with a redundant topology to automatically adjust the network's data forwarding path in the event of a failure. Cisco's PVST+ solves the problem of STP's inability to achieve load sharing. STP takes a long time to reconverge, typically 30.50 seconds, and RSTP and MSTP can solve the problem. This chapter focuses on the operation and configuration of STP, STP-protected RSTP, and MSTP technologies used on switches to prevent Layer 2 switching loops, and also discusses switch stacking.14.1 STP Overview14.1.1 Introduction to STP To increase the redundancy of a LAN, redundant links are often introduced in the network design, yet this can cause switching loops. Switching loops can bring about problems such as broadcast storms, multiple copies of the same frame, and unstable switch CAM tables, which have an extremely serious impact on network performance. STP (SpanningTreeProtocol) can solve these problems. STP (IEEE802.Id) blocks redundant paths that could lead to loops to ensure that there is only one logical path between all destinations in the network. A port is said to be in a blocking (Block) state when it blocks traffic from entering or leaving. Blocking redundant paths is critical to prevent switching loops. To provide redundancy, these physical paths actually remain, but are simply disabled to avoid loops. In the event of a network failure that requires a blocked port to be enabled, STP recalculates the paths, unblocks the necessary ports, and puts the blocked port into the forwarding state. BPDU (Bridge ProtocolData Unit) is the data frame exchanged between switches running STP function, which contains two types: one is ConfigurationBPDU, used for spanning tree calculation; the other is Topology Change Notification (TopTree).The other is TopologyChangeNotification (TCN) BPDU, which is used to notify the change of network topology. BPDU fields and their meanings are shown in Table 14-1.Table 14-1 BPDU Fields and MeaningByte Number Field Description2 Protocol 1D The value of this field is always 01 Version Version of STP (value is 0 when using IEEE 802. Id)