cisco ccna prep program

cisco ccna prep program.As the reader can see, the broadcast frame 2 contention will be cycled through the LAN, causing the switch to forward too many useless frames.degrades the performance of the network. A situation known as a broadcast storm. For example, if host A sends a unicast frame to another host, but that host never replies, it will cause the unicast frame to loop around the LAN (because no switch knows which port the host is connected to). In general, as long as there are redundant links in the network's Layer 2 topology, there will be loops at the link layer (or Layer 2), which can easily lead to broadcast storms.A second side effect of a Layer 2 loop is that it can cause the switch's MAC table to change around. In the above example, SW1 would incorrectly keep the MAC address table updated, constantly having ports G0/2 and G0/3 correspond to the MAC address of host A, because SW1 would constantly receive frames from those two ports with source MAC addresses of host A. Thus, whenever a frame is received from host A, SW1 will send it out through the wrong port, making the situation worse.A third side effect of a Layer 2 loop is that a host (e.g., Host E) will continue to receive numerous copies of the same frame cycling back and forth within the network. This is likely to mislead the hosts and trigger a high-level protocol failure.Spanning Tree Protocol (STP) can be turned on to avoid layer 2 loops. This section will introduce the basic concepts of STP. Over the years, STP has been tweaked in terms of functionality and performance to accommodate increasingly complex network environments. The basic function of STP is to create a loop-free logical Layer 2 topology. To do this, it is necessary to have the switch allow certain ports to forward traffic while blocking other ports from forwarding traffic. If the topology changes (for example, if a link goes down), the switches running STP work together to compute a new logical topology (a computation process called "reconvergence"), while allowing some ports to resume traffic forwarding to accommodate the new topology.Figure 1-25 shows what happens when the switch in Figure 1-24 is enabled for STP. At this point, SW3 has set port G0/2 to a blocking state and will not forward traffic through it. This prevents broadcast frames from cycling through the LAN. If the interconnect link between SW1 and SW3 is broken, the STP tells SW3 to set port G0/2 to the forwarding state, resuming traffic forwarding on the interconnect link between SW2 and SW3. STP also incidentally provides the link redundancy function.Figure 1-25 Example of Layer 2 Traffic Forwarding with STP EnabledAn STP-enabled switch performs the Spanning Tree Algorithm (STA), which creates a tree-like, loop-free logical topology within the LAN. Before exploring the basic operation of STP, the following concepts need to be clarified.28 Chapter 1 Network Protocols and Network Device BasicsBridge ID (BID): 8-byte ID, calculated independently from each switch. The first 2 bytes of the BID are the priority field, and the remaining 6 bytes are the MAC address of the switch (a port).